cjr198car Posted July 27, 2011 Report Share Posted July 27, 2011 Well I did the 7/8th 280Z master cylinder upgrade, but now my rear brakes slowly lock after a few miles of driving. What did I do wrong?! I did use heat on the factory proportioning block to get the old lines off (the lines would not come off the old MC). I used the old arm from the 510 MC and put that into the 280Z MC. My thoughts are: I over heated the prop block and now its not releasing pressure from the rear? The 510 MC arm is to long and not releasing pressure on the 280Z MC? I can tell you that when the rear brakes lock up the pedal becomes firmer and firmer and I can release the rear brakes by opening the rear bleeder on the MC. HELP ME OUT GUYS! -Colton Quote Link to comment
datzenmike Posted July 27, 2011 Report Share Posted July 27, 2011 Usually if the push rod is too long or not adjusted the fronts slowly lock first as the fronts are first inside the MC. Never heard of the backs locking like this. When they lock, loosen the front line at the MC and see if pressure is vented there. If so I would suspect the MC or it's adjustment. Maybe the piston jams and won't return. If it's OK try the line at one of the rear wheel cylinders. If pressure vents there then it's trapped between there and the proportioning valve. Be sure there is some play on the peddle. About 1/16" of peddle movement before the pushrod meets any resistance is fine. You have to remove the pushrod, loosen the lock nut and turn to shorten it. Quote Link to comment
thisismatt Posted July 27, 2011 Report Share Posted July 27, 2011 Are you sure you don't have the lines reversed at the MC? Quote Link to comment
cjr198car Posted July 27, 2011 Author Report Share Posted July 27, 2011 I dont think im explaining it right. Let me try again... The rear brakes literally start locking up and do not release after a few miles and the pedal gets firmer and firmer like its not releasing. Its like it has a line lock for the rear brakes. Im thinking the prop valve is messed up since I used a torch to get the fittings off. I dont believe the lines are crossed (I did not bend new lines and the lines lined up like they were installed before). I would bet if I release the front bleeder on the MC the rears would unlock also. Signs are pointing to the proportioning block Ive just never heard of anything like this happening. Does this shed anymore light on the issue? Does anyone have experience with eliminating the stock prop valve (possibly pics?) Does anyone have an oem proportioning valve for sale? Quote Link to comment
datzenmike Posted July 27, 2011 Report Share Posted July 27, 2011 I would bet if I release the front bleeder on the MC the rears would unlock also. Then the pressure is being held at the MC and not the proportioning valve. When they lock, loosen the front line at the MC and see if pressure is vented there. If so I would suspect the MC or it's adjustment. Maybe the piston jams and won't return. If it's OK try the line at one of the rear wheel cylinders. If pressure vents there then it's trapped between there and the proportioning valve. Do this to narrow it down. Quote Link to comment
cjr198car Posted July 27, 2011 Author Report Share Posted July 27, 2011 I release the bleeder at the MC (the rear bleeder, closest to the firewall) and it releases the rear brakes. That would suggest the MC is holding the pressure thus the issue is with the MC. I have free play in the rod and pedal. The stock 510 rod was to small for the 280Z washer and c clip so I put an additional washer on the rod so it would not pop out of the 280Z MC (only a few mm thick and it is not pushing down on the MC piston). Has anyone else experienced having to modify this part due to this issue? I at first thought the washer was acting as a shim and not releasing the brakes, but its not pushing on the MC piston. I also dont get why the pedal gets firmer and firmer as the brakes start to lock up in the rear. Im almost thinking after typing this the factory prop valve is messed up. Its like when you pump up the brakes to bleed them. I think the prop valve is holding pressure against the rear brakes, but somehow releasing pressure off the MC frees up everything for a few more brake pumps. Sounds like I need to try to release pressure at the rear discs, although I know pressure will come out and the brakes will release...hmmmm Does anyone have schematic of the stock proportioning block? Quote Link to comment
thisismatt Posted July 27, 2011 Report Share Posted July 27, 2011 I release the bleeder at the MC (the rear bleeder, closest to the firewall) and it releases the rear brakes. That would suggest the MC is holding the pressure thus the issue is with the MC. I have free play in the rod and pedal. The stock 510 rod was to small for the 280Z washer and c clip so I put an additional washer on the rod so it would not pop out of the 280Z MC (only a few mm thick and it is not pushing down on the MC piston). Has anyone else experienced having to modify this part due to this issue? I at first thought the washer was acting as a shim and not releasing the brakes, but its not pushing on the MC piston. I also dont get why the pedal gets firmer and firmer as the brakes start to lock up in the rear. Im almost thinking after typing this the factory prop valve is messed up. Its like when you pump up the brakes to bleed them. I think the prop valve is holding pressure against the rear brakes, but somehow releasing pressure off the MC frees up everything for a few more brake pumps. Sounds like I need to try to release pressure at the rear discs, although I know pressure will come out and the brakes will release...hmmmm Does anyone have schematic of the stock proportioning block? The reservoir, line, and bleeder closest to the firewall are for the front brakes. If you crack the rear brake bleeder (furthest from the firewall) what happens? Quote Link to comment
cjr198car Posted July 27, 2011 Author Report Share Posted July 27, 2011 "The reservoir, line, and bleeder closest to the firewall are for the front brakes. If you crack the rear brake bleeder (furthest from the firewall) what happens?" Thats odd? I havent done that yet. I think its extremely odd that I release the front brake bleeder and the rear brakes release... Does this point towards a messed up brake assembly switch (I was calling this a proportioning valve)??? Quote Link to comment
thisismatt Posted July 27, 2011 Report Share Posted July 27, 2011 "The reservoir, line, and bleeder closest to the firewall are for the front brakes. If you crack the rear brake bleeder (furthest from the firewall) what happens?" Thats odd? I havent done that yet. I think its extremely odd that I release the front brake bleeder and the rear brakes release... Does this point towards a messed up brake assembly switch (I was calling this a proportioning valve)??? It's odd that the rears are sticking on. Did you read my post before I edited it the first time? :P There is a brake failure indicator switch (the block that has a wire coming off it) to sense if either of your brake circuit fails. But, it looks like there may also be a proportioning valve on the 280z (there isn't on the 510, but there's some other block on the 280z that I'm not familiar with). If you crack the bleeder on the MC furthest from the firewall and it doesn't release the rears, then I still think you have the lines crossed somewhere. Either at the MC, the indicator switch block, or that 3rd block. Quote Link to comment
DISLEXICDIME Posted July 27, 2011 Report Share Posted July 27, 2011 if memory serves me right when you do the zx master swap you are supposed to use the zx push rod Quote Link to comment
cjr198car Posted July 27, 2011 Author Report Share Posted July 27, 2011 Ill have to check! Im using the 280Z not ZX MC. Quote Link to comment
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