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I will try to keep it short. Before I started my SBC swap, and rear disc conversion, and suspension rebuild, and... Okay right after I picked the car up and decided to rebuild the rear drums I bled the brakes and have had this issue since then.

 

After bleeding the system: M/C then RR RL FL FR (looking at front of car) The brake pedal was weak when the car was off and would just go to the floor when the car was on. I would get some pressure but not much. I could stop the car though. That was last year and I could not figure it out, blamed it on leaking rear cylinders and decided I was going rear discs anyways. I did not have this problem when I got the car at first... That I can remember considering it sat for 6 months lol.

 

FF to current day and after much tribulation my rear discs (280ZX) are installed. Bled the system as above but used a Motive power bleeder, which made everything go much faster. 

I still have the same damn problem!! I can push the pedal maybe 2-3 inches with slight resistance And then it gets very firm. I can pump as much as I want with no changes. I have bled the system at least 4 times in the past two days and tried bleeding through the M/C brake line fittings in lieu of bench bleeding. No changes.

 

I wedged a board between the brake pedal and seat when it got to the point of being very firm and went to test each rotor. the rears both spin with little resistance and the fronts were locked up. This mimics what was happening before I had discs in the rear. I am getting plenty of fluid out of each bleeder and yes the are both at the top of the calipers.

 

It really does not seem like I have air in the system as I cannot build pressure with pumping the pedal. This only leaves the M/C so I pulled it to see if fluid was leaking from the rear in to the booster but I did not see any. Also checked to see if the reaction disk was in place and it is. 

 

I ordered a new M/C but I am not confident that will solve the problem.

Any ideas?

Thanks

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For starters, it sounds as though your reaction disk has fallen out of position in the brake booster.  Search here for reaction disk repair...it's an easy fix, if that's what's happened.

 

Next, did you go with a bigger MC when you installed the rear discs?  I think most people choose a 1" MC when doing rear disc conversion....that might possibly be the cause of weak rear pressure.  (You may also find that you need an adjustable rear proportioning valve, once the swap is completed.)

 

Finally, if you still have the stock tee connector for the rear brake hard lines in the system, I would bleed the left rear first...then right rear.  The stock tee connector is on the passenger side of the car, which actually results in the left rear having the longest run of brake line of the four wheels.

 

Good luck with it.

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Posted (edited)
1 hour ago, Twisted46 said:

 blamed it on leaking rear cylinders and decided I was going rear discs anyways. I did not have this problem when I got the car at first...

 

I can push the pedal maybe 2-3 inches with slight resistance And then it gets very firm. I can pump as much as I want with no changes.

 

I ordered a new M/C but I am not confident that will solve the problem.

Any ideas?

 

 

If the rear cylinders were leaking there would be fluid.  Did you see any?

 

Sounds more like a bad MC.  The front and rear systems are stacked, in series.  If one seal fails the pedal will drop that distance until the second one catches.  Your new MC might fix it.

 

Here's a drawing, they're all the same.  The right seal contains the fluid, the other two create pressure.  You can see how you can lose one but keep pressure on the other end.

 

image.thumb.png.89850c207bd092751134a357b856d596.png

 

 

Edited by NewZed
Dang...
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Hey guys. Thanks for the quick responses. I checked the reaction disc and it is in place. 

 

I have a 7/8 M/C as i read it is sufficient with oem front calipers. I still have the 260z prop valve in though. The new M/C will be in today.

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Posted (edited)

Well I installed a new MC and bled everything. The problem just got worse.

 

Edit: I guess I should add more detail instead of throwing a fit and thinking about getting rid of the car again.

 

I installed the new MC and removed the inner workings of the OEM proportioning valve. I then used the pressure bleeder and started at the MC, I did not bench bleed the MC but I used the pressure bleeder on it in the car through all 4 ports, then did the corners. I did not see a ton of air coming out of the line but I did see some, LOTS of fluid movement from the pressure bleeder. The pedal feels even softer than before and now I can actually push farther than before.

 

Also as I bleed one reservoir the other one will start to rise and actually overflowed on my twice (one the front and once the rear). Could an internal seal be shot on this pile?

 

Edited by Twisted46

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Maybe try old school bleeding methods before the pressure bleeder.  As Miles suggests there might be air in the MC.  An air bubble might not get pushed out of the MC by a pressure bleeder if the MC is "nose up".  Be the air bubble...where would you hide?

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47 minutes ago, NewZed said:

Maybe try old school bleeding methods before the pressure bleeder.  As Miles suggests there might be air in the MC.  An air bubble might not get pushed out of the MC by a pressure bleeder if the MC is "nose up".  Be the air bubble...where would you hide?

 

^^Yep ^^

 

Like this..................................... 

 

 

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You gutted the OEM proportioning valve?

 

The 240Z PV can be gutted as it only effects the rear brake circuit. Not sure about the 260Z and 280Z PVs, but you may have just tied the front and back hydraulic circuits together.  Somebody chime in on this.

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I messed with it for another hour and still nothing. I need more brake fluid and patience to keep going. The new master is allowing fluid to travel between the reservoirs which does not seem correct. 

 

And I bench bled the ports but not the bleeders. I will try all 4 at once when I have enough hose.  

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3 minutes ago, Twisted46 said:

I messed with it for another hour and still nothing. I need more brake fluid and patience to keep going. The new master is allowing fluid to travel between the reservoirs which does not seem correct. 

 

And I bench bled the ports but not the bleeders. I will try all 4 at once when I have enough hose.  

 

 

See my last post. If you gutted the PV you may have now tied the front and rear brake circuits together which would explain fluid flowing between the tanks.

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It's a late 260Z right?  Here's a 1975 NP valve drawing.  What would be odd though is that, if it is gutted and all systems are on the same hydraulic line, if one wheel gets pressure they all should get pressure.

 

You might be working on the wrong problem.  Maybe it's mechanical.  

 

The beauty of the old school bleeding method is that you actually see the response at each wheel as you bleed it.  Fluid squirts out, showing that some pressure is being transferred.

 

image.thumb.png.3984c6f0fc6dc2d1229fc8307b531af2.png

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I am going to see if anyone from thr local Z club can come and take a look. I do not know enough about this brake system... Clearly.

 

Anyway I need more fluid and tubing tomorrow to try to bench bleed a third time to rule that out. I am also going to buy some line to remove the PV completely for peace of mind. 

 

The problem I have with old fashioned bleeding at the moment is my wife is pregnant and doesn't want to be around "all those chemicals" right now. When I get a friend to come over I was planning to do a final bleed that way.

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Posted (edited)
35 minutes ago, Twisted46 said:

The problem I have with old fashioned bleeding at the moment is my wife is pregnant and doesn't want to be around "all those chemicals" right now. When I get a friend to come over I was planning to do a final bleed that way.

 

I bleed mine using the stick between the pedal and seat.  Crack the bleeder, push the pedal and jam it down with the stick, close the bleed screw, let the pedal up.  Works surprisingly well.  Put a tube in a jar and you can see what came out.

 

I have speed bleeders on the back also.  Still drums.

Edited by NewZed

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Thanks again for all the input guys.

 

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) even though their own site specs them out. They are 35mm which looks to be the longest size for m10x1.

The ones Miles linked look like they go on rear cylinders. I have a set for the 280ZX rear calipers and they are actually the same size as the fronts.

 

Maybe Earls has longer ones for the front that will work? I order some m10 unions I plant to use to delete the PV completely for testing. This will all have to wait until Sunday though.

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Posted (edited)

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.

Edited by jhm

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Posted (edited)

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 by Twisted46

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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

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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.

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