Blowing mastercylinders? Or some other issue

[Markmartin01]

I'm having a heck of a time on a 91 Z71. It had issues with blowing wheel cylinders so I fabbed in a rear end from an 01 so it now has 4 wheel disc brakes. ABS was bypassed long ago, used original proportioning valve, no lock up in rear, braking was smooth all around. Then it lost brakes. Master cylinder was "whooshing" so i assume bad oring, replace. Then brakes are stiff/soft intermittently, so i replace the master cylinder again. Just a few days ago I replaced the master cylinder again, this time with an appropriate 01 style meant for rear disc. Works ok, but pedal has a bit too much travel and doesn't seem to really grab with confidence. I can't seem to really work out the kinks here so the brakes feel normal. Or at least normal for a 91 with later model rear discs. Any idea what to chase down first here? I've bled the brakes, bled the master, can't see any air trapped. Bit stumped.

 

[evilunclegrimace]

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You need to check this pin protrusion. GM has a pin height tool to do this, it is A Kent-Moore tool # 37839.

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[evilunclegrimace]

^^^^ This is also a must for anyone that is reconfiguring the master to a 4 wheel disc set up and complaining about pedal travel. If the pin is to far away from the master cylinder piston you will have excess pedal travel. Also if the pin is to far out it will not allow the master piston return completely and there will be a reduction in fluid flow and there is the possibility of damage to the piston seals due to travel beyond the usable master cylinder piston bore.

 

[Markmartin01]

@evilunclegrimace Good info, thank you. I checked the depth on the back of both mastercylinders and it was the same, but that doesn't necessarily mean the plunger depth would be the same. I did resolve the pedal travel issue, it was just that I had took a shortcut and put the bracket between the MC and the booster to keep from having to bend lines. I pulled the bracket completely off as it was only there to hold the now disabled ABS control and travel is fine. I do have a hard pedal now, almost like no boost. Booster is good though. I'm curious if I have overlooked a prop valve issue. I have read the prop valve has a piston that moves front or back if no fluid is detected to block off the empty reservoir. May have to pull lines on the prop valve and check the piston in it to see if it's stuck on one side. Brakes feel almost like they did when I blocked off the rear of the MC for a few days when it blew a wheel cylinder before dumping the drum brakes. The bleeder check valve on the prop valve also will not weep any fluid so perhaps it's blocked by the piston or air locked in some fashion.

 

[Markmartin01]

SO after digging a bit deeper on this site i found the thread at https://www.gmt400.com/threads/new-front-brakes-wont-lock-up-big-tires-the-problem.43840/ and someone else had been down the same road. So last night I went and picked up yet another MC, this time the appropriate for RCSB 4x4, 36mm bore. Apparently the GMT800 MC does not like the GMT400 small front calipers and it causes a hard pedal. We'll see if it holds this time because previously MC's have lasted a week or two and then everything gets sideways.