Front brakes grab on wet roads locking the front wheels

Tiny
MONGOOSE67
  • MEMBER
  • 1985 CHEVROLET SUBURBAN
  • 5.7L
  • V8
  • 2WD
  • AUTOMATIC
  • 100,450 MILES
Ever since I own this the front brakes grab on wet roads will actually lock the front wheels but release as soon as I take my foot off the pedal. I have replaced front rotors, front calipers, rear brake shoes were brake drums all new brakes new brake lines new master cylinder still the same issue. When I push on the brake pedal it goes down about two inches before it starts to apply the brakes. Pedal feels normal at this point it is not hard.
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Monday, September 19th, 2016 AT 5:55 PM

4 Replies

Tiny
CARADIODOC
  • EXPERT
This is not a common problem, but the first thing I would consider is the combination valve. That is a brass block that sits on the frame rail right under the master cylinder. It will have a single wire plugged into it in the middle, on top. Follow the steel lines from the master cylinder to this valve assembly.

The front valve is the "metering" valve, also called the "hold-off" valve. Its job is to require the brake fluid to reach about fifteen pounds of pressure to open that valve so fluid can flow to the front calipers. That is to give the rear shoes time to expand and make contact with the drums. It insures front and rear brakes start to apply at the same time. On some vehicles, older Fords in particular, that metering valve has to be held open by hand or with a little special spring-metal tool to allow for gravity-bleeding of the front brakes. When that tool is forgotten and is left in place, you'll get the real easy front-wheel lockup you described.

The stem of that valve sticks out in the front of the assembly and it might have a rubber cap around it. You should see that stem pop out a little further when a helper presses the brake pedal.

Has the ride height been altered in the rear or is it sitting higher than it is supposed to be?
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Monday, September 19th, 2016 AT 6:48 PM
Tiny
MONGOOSE67
  • MEMBER
No. It has stock height.16.5 tires. I did however notice last night when bleeding the brakes again it does not seem to have a lot of pressure in the back when you break the bleeder lose.
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Tuesday, September 20th, 2016 AT 4:17 AM
Tiny
MONGOOSE67
  • MEMBER
So are you possibly suggesting that brass block is bad?
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Tuesday, September 20th, 2016 AT 4:22 AM
Tiny
CARADIODOC
  • EXPERT
Yup. I described how to watch the metering valve.

The tires affect ride height of the body, but I'm referring to suspension ride height. That has nothing to do with tire size. It has to do with sagged springs. The symptom is different though. Most pickup trucks and minivans use a height-sensing proportioning valve in the rear because there can be such a wide range of loading. With a heavy load, the rear sags, and that valve sends more braking power to the rear wheels. More accurately, under light loading, the valve limits the amount of brake fluid pressure to the rear wheels to prevent easy rear-wheel lockup. Sagged springs mimic heavy loading so you get more braking power to the rear than you should. That results in easy rear lockup, especially on wet roads. I was asking about your ride height because any modification that raised the rear would reduce rear braking power and make the front brakes do more of the work.
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Wednesday, September 21st, 2016 AT 2:39 AM

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