c5 Brake Issue
Thread Starter
Heel & Toe
Joined: May 2018
Posts: 21
Likes: 10
From: Stuart, FL
2023 C4 of the Year Finalist - Unmodified
c5 Brake Issue
Just bought a clean low mileage 1998 Pace Car. Only issue was a solid brake pedal which slowly depressed to the floor when stopped while the brakes were applied. No codes or warning lights displayed. No big deal, I thought. Replaced the master cylinder with a genuine GM part and fully bleed the system. Issue still there. Research indicates that it could be the Brake Pressure Monitoring Valve (Part # 12527809), but if that was the case it should have thrown a code. Anyone have any ideas? The BPMV appears to be a discontinued GM part and if one can be found sellers are asking $2k to $3k for them. Yikes. All of a sudden a good deal don't seem so good! Thanks
Drifting
Joined: Jan 2015
Posts: 1,256
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From: Pennsylvania
I would aggressively activate the abs multiple times and see if the problem changes. This is easy on wet roads just hammer the brakes and intentionally try to lock the tires up multiple times. This will do a couple things it will exercise the module and if there is a problem it should throw a code. You may also have some air in the module that will not move during normal bleeding but after activating the module you may be able to get the air out.
worst case scenario you can always do a abs delete
worst case scenario you can always do a abs delete
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Instructor
Joined: Aug 2022
Posts: 243
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From: Ashburn, VA
Brakes!!!! My Specialty. Sorry about your brake woes my dude. Whenever I run into an issue like this you gotta start at the rooter...and work to the tooter 
. Get 2 jugs of DOT 3/4 and lets begin:
I'm a brake snob and with a car like these, you want them to stop when commanded to. If you try the above and come up empty...I HIGHLY recommend one of those MOTIV power bleeder setups with multiple caps. Pressurize the pot WITH NO FLUID IN IT...say about 10-12 psi. Let it sit for like 10 minutes. If you got a leak or a pin hole it a line...it will sniff it out in no time. I've saved SOOOOOOOO MUCH TIME and headaches with the MOTIVE when doing brake work.
Hopefully all this helps. Good luck man
Joe
. Get 2 jugs of DOT 3/4 and lets begin:- Your master is new...I presume you bench bleed it before install...but I like to ask when diag'ing cause it eliminates misunderstandings. IF YOU DID...Unplug the front and rear ports and plug them with the plastic port bleeder cap thingy's. I have several fittings of various sizes with pinched off lines for this purpose but the main thing is you want to plug up the ports solid. Return to the cockpit and depress pedal...it should be harder then a teenager in a ***** house lol. The main thing to observer here is that the pedal should NOT MOVE...It should be rock ******* solid. If so, move on to #2. If not, you got air in the master...Time to bench bleed again.
- Checking the EBCM lines and module is kind of difficult without a bi-directional scanner which can activate the ABS pump. If you dont have one of these, the suggestion of a finding a gravel road and locking up[ the brakes to cycle the pump is a good idea. After doing so, re-bleed the system and see if your pedal gets better.
- What you can describe can ALSO be caused by weak aged flexing brake hoses. Pretty simple concept...they dont leak, but flex when brakes are applied...which gives a mushy pedal. Inspect the hoses. You can clamp each side off *GENTLY* with vice grips and a rag. and do the test in #1. When you find the one that the brakes dont sink...you've found your culprit. You could also be apply logic in #1 and clamp all 4 off and see if you have the same sinking pedal. This aint the most scientific procedure, but works pretty well if one brake hose is the culprit.
- I doubt you have a leak in the Calipers (as you'd see that with fluid on the ground) but rebuilding those puppies ain't a bad idea ESPECIALLY if you dont know the service history of the car.
I'm a brake snob and with a car like these, you want them to stop when commanded to. If you try the above and come up empty...I HIGHLY recommend one of those MOTIV power bleeder setups with multiple caps. Pressurize the pot WITH NO FLUID IN IT...say about 10-12 psi. Let it sit for like 10 minutes. If you got a leak or a pin hole it a line...it will sniff it out in no time. I've saved SOOOOOOOO MUCH TIME and headaches with the MOTIVE when doing brake work.
Hopefully all this helps. Good luck man
Joe
Last edited by averagejoesautos; May 5, 2023 at 08:48 AM.
Thread Starter
Heel & Toe
Joined: May 2018
Posts: 21
Likes: 10
From: Stuart, FL
2023 C4 of the Year Finalist - Unmodified
Thanks Joe. I did do the panic stop exercise with no changes. The MC was bench bled before install and the entire system has been power bled multiple times. I got it in my head that I may have gotten a defective "Genuine GM Part" MC. I am going to try your suggested of blocking off the ports test this weekend and if the pedal is not rock hard its off to the GM dealer for a replacement. I really am hoping that is the issue as the car is a real survivor. 18k miles and stored indoors all is life. I do not want to think about hunting down a EBCM or BPMV as they are hard to find and very expensive. Thanks again. I will let you know how it goes.
Instructor
Joined: Aug 2022
Posts: 243
Likes: 93
From: Ashburn, VA
Thanks Joe. I did do the panic stop exercise with no changes. The MC was bench bled before install and the entire system has been power bled multiple times. I got it in my head that I may have gotten a defective "Genuine GM Part" MC. I am going to try your suggested of blocking off the ports test this weekend and if the pedal is not rock hard its off to the GM dealer for a replacement. I really am hoping that is the issue as the car is a real survivor. 18k miles and stored indoors all is life. I do not want to think about hunting down a EBCM or BPMV as they are hard to find and very expensive. Thanks again. I will let you know how it goes.
