No (Low) Rear Brake Pressure

[Twisted46]

I have Dorman 12706 and Z Car Depot 27463 (The Z car bleeders are in a Russell box and IIRC 560 is what they say.) I will double check when I get home. They look identical to the Dormans i bought for the rear. Both are 35mm and the threaded portion is too short for my calipers on the front. Which may not be original I will take a picture when I get home. I bought a 40mm normal bleed screw from the parts store to test and it worked great so regardless the 35mm bleed screws are too short for my calipers. 

 

Update:

Look like Earls also offers 35mm as the max length in m10x1.

The Calipers on my front are Sumitomo and most likely were replaced at some time so maybe they use a deeper port? Ill take a picture of the bleed screw difference tonight.

Edited May 2, 2019 by Twisted46

[Miles]

2 hours ago, jhm said:

The correct Russell P.N. for the fronts is #639560.  I have them in my car and they work fine.  Do you recall what part number yours are?

 

IIRC, the MC uses the same size as the stock rear drums.

 

Yep, p/n 639560 for the front calipers

 

https://www.ebay.com/itm/Russell-Brake-Caliper-Speed-Bleeders-Front-Rear-for-1970-1973-Datsun-240Z-/192230009783

[ZHoob2004]

I have 639560 and 639570 on my car, not sure which one was which.

[Twisted46]

639570 is for the drum cylinders. I have 280zx rear calipers that use the same bleeder as the own fronts. The problem is that 639560 is too short for my fronts and I can't find any longer speed bleeders that are m10x1

[NewZed]

5 hours ago, Twisted46 said:

I do have speed bleeders for all 4 corners.

The problem is that the ones I bought for the fronts (Russell) ended up being too short (thread all the way in with out seating so they leak)

 

Do you have the leaky bleeders (eeewww...) on the car now?  How are you using your pressure bleeder, since the speed bleeders have a check valve inside?

 

Might be a good idea to get primitive.  Reinstall the stock bleed screws, get a foot long piece of hose and a jar, and do it the old-fashioned way.

[Miles]

Do your speed bleeders work with your Motive power bleeder ?

[Twisted46]

I have regular bleed screws on all 4 corners right now. I replaced the fronts right away when they were leaking with hardly any fluid in the line and no pressure at all... lol... and I replaced the rears to use the Motive as it does not work with speed bleeders. The plan was to put the speeds back in the rear before the next track day but it sucks I won't have fronts.

 

Sunday I am going to take the MC out for a 4th time and bleed it, delete PV with two line unions, and bleed the system again.

 

I just want to drive my V8 beast 😩

[Miles]

one more thing to check.

 

There is a push rod between the booster and the MC piston.  The rod is adjustable.  When you change MCs you have to adjust the push rod because the piston socket depth is different between the 7/8 MC and the 15/16 MC. Look it up.

 

If the push rod is  adjusted too long the brakes will lock up because the longer push rod prevents the piston from retracting all the way which blocks the fluid return port inside the MC.  This will result in your brakes locking up after  a few stops. If this happens while road testing just open the MC bleed screws and limp home and readjust the push rod.

 

If the push rod is too short you will experience a long pedal because the piston is not pushed far enough to fully engage the brakes. That is, the piston is not pushing enough fluid. Your brakes will feel mushy and you will experience a pedal with long travel before engaging the piston

 

Do a search for push rod adjustment. It is critical to properly functioning brakes and brake feel.

 

 

Edited May 2, 2019 by Miles

[Twisted46]

Thanks Miles,

I will search and check that too. I did note that the MCs looked nearly identical to each other before installing but who knows. I have spent no less than 20 hours on brakes, is suppose it is on of Z right of passage? like the spindle pins..... and everything else lol.

[Twisted46]

Okay so I am at least back to ground zero instead of being in the negative. I now have my new MC feeling like the old one (the pedal get firm at the bottom and does not go to the floor). The problem is still that I get a lot of travel before much happen. I noticed while bench bleeding (the way that Miles posted) the first of the two piston strokes is about half the resistance of the second, if that makes sense. I put about two quarts of fluid through it at this point and the pedal does not get any more firm with pumping so I do not think it is air. Two M/Cs with the same issue... Maybe the booster is bad?? the reaction disk is in place. 

 

The other issue of PV was solved quite nicely with two unions, no more issues there. 

 

I am going to to try and find someone local who knows S30s pretty well and have them take a look, I can't think of anything else. 

[Miles]

Again I ask, what size MC did you install?

 

Calipers require a larger volume of fluid.

 

So the pedal goes low because the MC has to pump more fluid.

 

You may have to install a 15/16 or  1 in. MC

Edited May 6, 2019 by Miles

[Twisted46]

It is an OEM 7/8 but it feels exactly the same as it did with drum brakes. This started when I first bled the system after buying the car and has existed since.

[jhm]

Pretty much everyone here has suggested trying a larger MC.  You have discs in the rear, and apparently, some non-stock calipers in the front.  It's very possible that the stock 7/8 MC will not handle your current setup.

[Twisted46]

I am certainly not ignoring anyone's suggestion to use a larger MC, but if you read back through the posts it was barely mentioned. However before I spend $150 (which is honestly a drop in the bucket at this point) on a new Wilwood 1" MC I would like to confirm that there are no other issues because again, I was having this problem with stock drums and the current front Sumitomo calipers.

 

Stage - 1The front calipers were in good shape when i bought the car so I did not replace them, they are still working great and lock up the front wheels when asked to do so. The pedal felt normal and firm when I bought the car.

Stage - 2 I bled the brakes because for all I knew the fluid was a decade old, problems with the soft pedal started.

Stage - 3 I then spent the next year fussing over this issue including replacing the rear drums with calipers because I am an idiot and hate drum brakes for no reason. No improvement

Stage - 4 The pedal feel got way worse when I replaced the MC last week and I could actual push the pedal to the floor without hassle.

Stage -5 Full bench bleed and system bleed this weekend at least put the pedal back to feeling like stage 2/3.

 

I am saying all of this because the problem happened only after I bled the stock brakes and before I started replacing anything.

[Miles]

On 5/2/2019 at 1:08 PM, Twisted46 said:

Thanks Miles,

I will search and check that too. I did note that the MCs looked nearly identical to each other before installing but who knows. I have spent no less than 20 hours on brakes, is suppose it is on of Z right of passage? like the spindle pins..... and everything else lol.

 

Please post a picture of your front calipers.

 

 

[JMortensen]

FWIW, I had a brand new brake system with new calipers, hard lines, soft lines, and dual masters that wouldn't bleed. Tried by myself, with help, with a mity vac and couldn't get a good pedal. What finally worked for me was using a 60cc food syringe and putting the fluid in through the bleeders at the calipers. I think there was a bubble that just wouldn't come out of the masters no matter what I did, but pushing the fluid backwards dislodged it.

 

[Miles]

8 hours ago, JMortensen said:

FWIW, I had a brand new brake system with new calipers, hard lines, soft lines, and dual masters that wouldn't bleed. Tried by myself, with help, with a mity vac and couldn't get a good pedal. What finally worked for me was using a 60cc food syringe and putting the fluid in through the bleeders at the calipers. I think there was a bubble that just wouldn't come out of the masters no matter what I did, but pushing the fluid backwards dislodged it.

 

I have done the same thing bleeding calipers  and master cylinders. Works.

 

The other thing some members have had to do was to remove the caliper while connected to the brake hose and shake the bubble towards the bleeder while bleeding. Seems to be a problem where the bleeder is not positioned vertically.  This is where a power bleeder comes in handy. Just be sure to block the piston in the caliper so it doesn't get pushed out. If the piston pops out you will be sad.

 

I am now wondering if he has non-stock calipers on the front.
 

 

 

Edited May 7, 2019 by Miles

[grannyknot]

It sure sounds like you have some air bubbles hung up in the system, like JMortensen I've used a large syringe to push the fluid backwards and sometimes it is the only method that will work but mostly I use a hand held vacuum pump to put 20 in.HG of vacuum on the line then when your helper has the pedal pressed firmly crack the bleeder valve for 2 sec. (you don't want your helpers foot go all the way down) then repeat.

You could also try tapping the caliper with a hammer while the bleeder valve is open to help dislodge the bubbles.

[NewZed]

Start at the pedal and figure out how each piece works.  You're talking about the booster but nobody uses the booster when they bleed the brakes, it doesn't matter.  Nobody has the engine running.  You haven't confirmed that your pedal play is correct, as described in the FSM.  

 

At this point, by your description, it sounds like the brakes work fine but the pedal throw is too long.  That sounds like a simple clevis adjustment at the pedal.  Study how the pedal presses on the rod that presses "through" the booster to the MC.  Make sure that all of the mechanical play is set correctly.

 

" (the pedal get firm at the bottom and does not go to the floor). The problem is still that I get a lot of travel before much happen. "

[Twisted46]

Hi All,

 

Thanks again for the helpful feedback and suggestions.

 

I probably will not have the car off the ground again until this weekend so I can't get a good picture of the caliper, all of the ones I have are not detailed enough. However I did replace the pads with a set that is specd for OEM use and they fit perfect but I will get pictures and part numbers.

 

I will try bleeding again (when I get back to working on the brakes) with the above mentioned reverse flow and removing the calipers (front and rear) to hold the bleeders vertical as the highest point. I could see the orientation of the rears allowing a small air pocket at the top.