Need help with dragging rear disc brakes.

[RebekahsZ]

"While I was at it," I did a rear disc brake conversion (85 maxima calipers) on my 240z in conjunction with my LS swap. LS is running great, but still having trouble with rear disc brakes. Made it more complicated by adding a second caliper for a "drifting hydraulic handbrake" what was I thinking??? Car feels a little heavy (draggy) and front brakes are staying cool (30 degrees celsius) while rears heat up (cruising down cool country road with little braking).

 

Anyway. I am going to totally disable the hydraulic hand brake and remove those calipers and their rubber flex lines. That at least limits me to one set of calipers.

 

What I have done already:

1) verified stock master cylinder push rod.

2) Loosened parking brake cable until it has slack

3) Turned wilwood adjustable proportion valve in the maximum "less brake" direction

4) Used a laser pointer type thermometer to check heat of rotors and calipers. Both the main and the drifting caliper are staying about the same temp on each wheel-sometimes i initially get a hotter reading on the driver side hydraulic parking brake caliper. The driver side is hotter.

When I stop the car, jack it up, the wheels turn with just the slightest drag on the brake pads-everything seems normal..is the same with motor on and off (vacuum booster is hooked up the LS2 intake manifold.

 

I want as many suggestions as possible, so please go crazy so I can find what I'm overlooking.

 

Two questions to open the floor:

1) Do the little tin pieces that clip into the calipers have any function? I can't see any, but maybe they help return the shoes? They seem to do nothing. See attached photo.

2) Will a stuck caliper on one side of car make the brakes drag on the opposite side as well?

 

Thanks. Keith

[Cannonball89]

Stock 240z master cylinders have a residual pressure valve in the rear circuit. This was installed to keep slight pressure against the drum brakes at all times. When you do a rear disk brake swap, the residual pressure valve must be removed, or you will have the slight dragging that you are describing. The valve is in the assembly under the rear reservoir.

 

Here is a picture from zcarnut's 1" master cylinder thread. The ones in the stock 240z master cylinder look similar and are in the same place, but there is only one in the rear circuit, not the front.

 

 

I hope this solves your problem, and while your at it, you may want to make sure that the handbrake master cylinder you installed doesn't have a residual pressure valve too.

 

P.S: You may want to think about upgrading to the 1" master or at least a 15/16" master. I have 280zx rear disks on my 240z which are a pretty similar piston size to the maxima's you have, and with the stock 7/8" master the brakes are very "squishy". I liked the firm pedal feel that the drums had, but the disks stop much better now that I have dialed them in with a prop valve. A larger master would bring back the firm pedal feel that the drums had I think.

[Aero Z]

Can you verify that the 280zx brake master had these residual pressure valves? I believe he has the 280zx master brake cylinder. I have the same 280zx master in my car too so I would like to know about this. When I have bled the rear brakes there is no fluid coming out of the bleed screw when the brake pedal is not pressed. Also the 280zx had rear discs so I wouldn't think that would be the case but would like it confirmed if anyone knows.

 

question 1: I always thought those tin clips are just to allow a smooth surface for the 'ears' of the pads to slide on so they don't have to machine a smooth surface on the brake caliper housing. As long as they're secure and straight they shouldn't be causing your dragging.

 

 

Stock 240z master cylinders have a residual pressure valve in the rear circuit. This was installed to keep slight pressure against the drum brakes at all times. When you do a rear disk brake swap, the residual pressure valve must be removed, or you will have the slight dragging that you are describing. The valve is in the assembly under the rear reservoir.

 

Here is a picture from zcarnut's 1" master cylinder thread. The ones in the stock 240z master cylinder look similar and are in the same place, but there is only one in the rear circuit, not the front.

 

 

[Cannonball89]

Oh, he mentioned that he "verified stock master cylinder pushrod" so I assumed he meant that he had the 240z master. And yes, a car with disks like the 280zx would not likely have residual pressure valves.

[RebekahsZ]

Thanks so much, guys. Anybody who has had a stuck caliper, please describe your symptoms so I can think about that some? While the temp of both rear rotors was twice the temp of the front, the left rear has consistently been the hotest. I will check for a residual valve-it would be so sweet if it was that simple. I love this forum!!! My master is a 15/16", so I assumed it was from a disc equipped car. The only way to know is to tear it down and check. Will probably be Wednesday before I can do it an get feedback posted. I have the stock 240z pushrod installed on my 15/16 master cyl. I have not yet adjusted the little rod that connects the booster to the master cylinder. If I remember correctly, it is adjusted to the short side, not the long side: could that be a problem? There is a lot of travel before the pedal firms up; I assumed that was due to needing to lengthen that little pushrod that connects the booster to the MC. I was saving that until I don't have dragging brakes. Thanks guys, I really appreciate it.

Edited March 12, 2012 by RebekahsZ

[beermanpete]

Thanks so much, guys. Anybody who has had a stuck caliper, please describe your symptoms so I can think about that some? While the temp of both rear rotors was twice the temp of the front, the left rear has consistently been the hotest. I will check for a residual valve-it would be so sweet if it was that simple. I love this forum!!! My master is a 15/16", so I assumed it was from a disc equipped car. The only way to know is to tear it down and check. Will probably be Wednesday before I can do it an get feedback posted. I have the stock 240z pushrod installed on my 15/16 master cyl. I have not yet adjusted the little rod that connects the booster to the master cylinder. If I remember correctly, it is adjusted to the short side, not the long side: could that be a problem? There is a lot of travel before the pedal firms up; I assumed that was due to needing to lengthen that little pushrod that connects the booster to the MC. I was saving that until I don't have dragging brakes. Thanks guys, I really appreciate it.

 

The adjustment between the booster and the master could cause the brakes to drag. There needs to be free play between the two to ensure that the master cylinder pistons return all the way and open the vent holes into the reservoir. If these vent holes are closed the brake lines are sealed when you are not using the brakes. As the fluid heat up it expands and applies the brakes. When the vents are open the fluid will vent into the reservoir as it expands without building any pressure.

 

Press the brake pedal slowly with your hand. You should be able to detect 2 distinct points when free play is taken up. Once when the push rod between the pedal and booster touches, and again when the booster push rod touches the master cylinder piston. If you cannot detect 2 distinct contact points you should verify the free play between the booster and the master by measuring the length of the push rod and the depth of the cavity it goes into. I believe the factory manual explains the procedure.

Edited March 12, 2012 by beermanpete

[RebekahsZ]

I did observe one other thing, but I don't know if it is truly notable. The brake fluid in my reservoirs is much darker color (honey) than it used to be (almost like water). It used to be clear...what's up with that? The brake fluid stays cool in the reseroir, surely any heat isn't making it into the master cyl? I haven't even trompped hard on my brake yet, I've been babying it...no panic stops.

 

I'll feel around for those two distinct stops on the linkage.

 

Reading lots about reaction discs and excessively long booster to master push rods. Would that cause my problem - which is limited to the REAR brakes? When driving, it actually drives and brakes pretty nice.

 

11:00pm. Been searching and reading-thank goodness for this forum. I went out and loosened the little bumper bolt that keeps the pedal from returning all the way up. I can now feel two distinct points at which things change. I am assuming that this is the pedal pushrod hitting the booster pushrod, then the booster pushrod hitting the master cylinder. Now that I have a little slack in the system, I will test drive tomorrow (straight pipes at midnight! makes the neighbors hate you). I have started a build guide-I will include a chapter about all the adjustments that are necessary to actually get the car on the road. I used to be really happy with the way my pedals were adjusted in a straight line so that lifting a heel was not necessary to move from pedal to pedal-that is all screwed up now.

Edited March 13, 2012 by RebekahsZ

[RebekahsZ]

Look what I found! A check valve in my master cylinder! Cannonball89: you are 'da man!!! Going out to bleed brakes. Will report back after a test drive.

 

Something else I found that isn't pertinent to this problem, but interesting. The brake fluid in each of my reservoirs has turned a different color. The one from the hydraulic handbrake (new Willwood MC) is still clear. The fluid in the reservoir from my old 15/16" brake MC has discolored honey-color. The fluid in my clutch master cylinder (new Tilton MC) attached to 13k-mile LS2 slave has turned almost black. The same DOT3 fluid was used to fill all three. Is this a heat thing, or is it just do to dirty fluid in the used portions of each system working its way back into the reservoir? I bled them all until clear at the bleeder screw! Any thoughts from the forum?

 

Also, is there a safe dye that I can add to the brake fluid to make it easier to see in the reservoir when it is clean? I actually check this stuff when I open my hood and it would be easier to see that I have lost no brake fluid if I could color it (assuming this natural discoloration stops).

[JMortensen]

Just get some ATE Super Blue fluid. It's good fluid and it's blue.

[Cannonball89]

Glad you got it fixed. And I think that the discoloration of the fluid is normal. The fluid in my car turned a brownish-black color too until I put a new master cylinder, rebuilt calipers, a new proportioning vavle/distribution block, and new brake lines. So in other words, unless your going to install all new brake hardware, you can expect discoloration.

 

Plus one for ATE super blue, it is what I use in the racecar.

[RebekahsZ]

Thanks to everybody! Getting the check valve out was the ticket. I wonder why my disc swap kit instructions didn't mention this little tid-bit. I've been around Z-cars for 20 years and had never heard of it, and I've read lots of threads on disc brake conversions!!! It is strange, the pedal is higher too, now that the check valve is out....I've even watched Z-car disc swaps on youtube...?!

 

I turned my proportion valve in the direction to put minimal brake pressure to rear wheels. I drove my car around the block and hammered the brakes a few times to see that the front brakes locked first. No pulling to either side. Turned proportion valve in direction to increase pressure to rear wheels-repeated hard stop procedure and car stopped like a bandit with no pulling and no tires locking up. I will repeat this sometime when the streets are wet to really fine-tune my bias based on worst case traction scenario. I'm hoping, for the most part, to set it and leave it.

 

Used laser pyrometer when I got back to my garage. All 4 rotors were about the same temp (160 degrees F). The hydraulic handbrake pads were the same temp as the rear rotors. However, the left footbrake pad was about 200 degrees F, while the right footbrake pad was about 160. Why would this one pad be so much higher temp? Should I worry about it? I picked up new (rebuilt) calipers today, so I can switch one if that is the problem.

 

I'd really like to hear what some of the enduro racers do to test and tune their brake systems to ensure reliability. Any pointers appreciated-I really want reliable brakes before I start racing (hopefully in 2 months).

 

I'll do a google search for the ATE brake fluid to find a supplier.

[RebekahsZ]

Oops-double post while trying to edit.

Edited March 19, 2012 by RebekahsZ

[JMortensen]

Because you have less weight transfer when it is wet, you can actually use more rear braking. So set it for dry and then up it for wet if you want. I like the way Neil Roberts suggests setting bias, some others here have taken issue with it, but as I said in that thread I kinda lucked into his setting and it worked fine for me.

 

http://forums.hybridz.org/index.php/topic/99500-tail-wagging-under-hard-braking/page__p__932930__hl__%2Bbrake+%2Bbias+%2Broberts__fromsearch__1#entry932930

[Aero Z]

What master cylinder are you using that had those check valves in it?

 

I have also used ATE superblue in another car I had. You can get it thru Amazon.

 

Look what I found! A check valve in my master cylinder! Cannonball89: you are 'da man!!! Going out to bleed brakes. Will report back after a test drive.

 

[Leon]

Thanks to everybody! Getting the check valve out was the ticket. I wonder why my disc swap kit instructions didn't mention this little tid-bit. I've been around Z-cars for 20 years and had never heard of it, and I've read lots of threads on disc brake conversions!!! It is strange, the pedal is higher too, now that the check valve is out....I've even watched Z-car disc swaps on youtube...?!

 

I turned my proportion valve in the direction to put minimal brake pressure to rear wheels. I drove my car around the block and hammered the brakes a few times to see that the front brakes locked first. No pulling to either side. Turned proportion valve in direction to increase pressure to rear wheels-repeated hard stop procedure and car stopped like a bandit with no pulling and no tires locking up. I will repeat this sometime when the streets are wet to really fine-tune my bias based on worst case traction scenario. I'm hoping, for the most part, to set it and leave it.

 

Used laser pyrometer when I got back to my garage. All 4 rotors were about the same temp (160 degrees F). The hydraulic handbrake pads were the same temp as the rear rotors. However, the left footbrake pad was about 200 degrees F, while the right footbrake pad was about 160. Why would this one pad be so much higher temp? Should I worry about it? I picked up new (rebuilt) calipers today, so I can switch one if that is the problem.

 

I'd really like to hear what some of the enduro racers do to test and tune their brake systems to ensure reliability. Any pointers appreciated-I really want reliable brakes before I start racing (hopefully in 2 months).

 

I'll do a google search for the ATE brake fluid to find a supplier.

Brake bias is not a "set and forget" kind of deal, this is why cockpit-adjustable proportioning valves exist.

 

As previously alluded to, weight transfer changes depending on the level of grip. Production car brakes will be set to best grip scenario, i.e. in the case where you have the highest amount of weight transfer to the front. Thus, the brakes are tuned to be heavily biased towards the front. Tuning the brakes to best-grip is safest, since the fronts will always lock first, no matter what surface you're on. If you tune to wet conditions, you will have too much rear bias for when it's dry. This causes an unstable condition since you'll be locking the rears before the fronts and going for a ride on the carousel.

 

Therefore, you must tune the brakes with the same grip conditions as you plan to experience on the track. Ideally, you'd be tuning at the track, but as long as you're using the same tires and similar ambient conditions, you should be close. If you want to set your brakes for the wet, you'll have to go through the same procedure, increasing rear bias until the fronts lock just before the rears. This gives you two distinct rear bias settings, "dry" and "wet".

[Cannonball89]

What master cylinder are you using that had those check valves in it?

 

I have also used ATE superblue in another car I had. You can get it thru Amazon.

 

I wonder if someone fitted a 15/16 master cylinder to a car with drums and added the check valve themselves?

[RebekahsZ]

I can't remember, it has been too long, but I may have purchased this from an ebay Datsun parts seller who MAY have advertised it as a direct replacement for the stock master cylinder. So, he may have added the check valve. I am just so happy that Cannonball gave me such an easy solution - once I started into it, I was about an hour between starting the repair to being on a test drive. I'd have never known to look for a check valve without his suggestion, and I'd never have seen the check valve if he had not sent me a picture. How did we ever live before the internet?

 

I will be doing all dry-weather racing, for sure. I will try all the tuning techniques that have been suggested. Thanks guys.

[inZane 240]

I have the same problem. Did you remove both check valves in the master cylinder or just the one for the rear circuit?

[Cannonball89]

Stock master cylinders only had it in the rear circuit for the drums. The one in the picture I posted is an aftermarket wilwood 1" master that I used just because I didn't have any good pics of a stock one.

[RebekahsZ]

I bet many of us who have done this "upgrade" have the same problem. I was only dragging in the back (even after driving around for 3-4 miles my front brake rotors were no hotter than air temperature, while the rears were smoking and stinking and making the car seem like it was losing more power by the mile), so I only looked for a check-valve in the portion of the master cylinder housing marked "R." I was able to do this all with the master cylinder still in the car. I used a syringe to suck the reservoir dry and put a lot of rags all over the place to try to protect my paint. Then, I disconnected the proper line, then remove the larger fitting that screws into the master cylinder (17mm wrench is too loose, 11/16 fit pretty good). I used a mirror to look up into the master cylinder and there I could see the male part of a flared fitting. I carefully pried the male tapered fitting out, trying not to mar the tapered part. It popped out and the check valve and spring came out right behind it. I put the male tapered flare back into the master cyl and secured it with the larger "banjo" nut and installed the line. Bled the master cylinder (it had taken a LOT of air) by filling it, and recirculating the fluid back into the reservoir until no bubbles came thru the clear line I was using. I went back and bled the calipers (the angle makes a big difference, so take of the calipers, jam a piece of wood between the pads to mimic the rotor, and bleed them while holding them at different angles until no bubbles escape. Put it all together and go for a test drive. All 4 rotors warmed to about the same temperature. I have no idea what temperature is "normal." It probably depends on how much braking has been going on. I want to drive around on a straight highway to cool the brakes, then coast to a stop and check relative temperatures before I declare this project done. I bought some rebuilt calipers to try if I determine that a caliper is dragging. Then I'll tweak front to rear bias once I have a muffler.

Edited March 20, 2012 by RebekahsZ