rear brakes locking up???

[Splorer Man]

i have a 94 XLT/5spd with anti lock brakes,they have been working good till yesterday.On the way to work i have to stop a few times,then i went to pull into the coffee shop and the rear wheels lock up :fire:.I unhooked the ABS and still the same.The front does work but the rear grabs more.Any ideas???

 

[MANZ]

I have the exact same truck and the exact same problem with my brakes, even after replacing almost every brake system component. Help!!

 

[Splorer Man]

apperently you and me are the only ones with this problem.My brakes are wicked touchy,if i have to stop quick the back lock up.Aint gonna be pretty this winter.Did you replace your proporsioning??? valve??

 

[TheJMan]

I have the exact same truck and the exact same problem with my brakes, even after replacing almost every brake system component. Help!!

I have a 1993 with the 4-wheel ABS and I too get some rear-wheel lockup under heavy braking. However, I flushed the brake fluid with Valvoline Synpower brake fluid, and since then, the rears haven't locked up quite as easily as they used to.

Another thing to consider...

If your shocks are worn out (namely the front ones) it could be making the transfer of weight during braking very drastic which would cause the lockup. I've got Monroe Reflex all around on mine and they have some special built-in valve that senses quick cornering or heavy braking and stiffens the shock automatically. They are very nice and well worth the investment.

 

[410Fortune]

check the vacuum line to the brake booster for fluid

Flush and bleed the system, reset the ABS computer.

See if it still happens.
If it does then the stock proportioning is likely set to much to the rear, it can be adjusted, but as a last resort.

 

[MANZ]

JMan - Thanks. I have new shocks (Reflex) and have flushed the system numerous times. I have new rotors, bearings, pads, drums, shoes, all new hardware. This is getting old. It feels like the proportioning valve is sending way too much pressure to the rear brakes. Any idea where the valve is on a '94? I thought it was internal with the ABS but I could be mistaken.

 

[MANZ]

410 -

I see I spoke too soon - I guess it does have a valve after all. Thanks for the tips, I'll let you know what I find.

 

[410Fortune]

I dont believe the 94 ABS system has a bleeder on it. The only bleeders I have seen are on the RABS equipped trucks.

The master cylinder is adjustable, at the booster plunger, but it is a very touchy process and is set from the factory.
If the rears are locking up the problem may lye elsewhere (booster, suspension, etc)

 

[MANZ]

apperently you and me are the only ones with this problem.My brakes are wicked touchy,if i have to stop quick the back lock up.Aint gonna be pretty this winter.Did you replace your proporsioning??? valve??

My brakes do the exact same thing. I have read plenty of threads where other folks have the same problem. I have not replaced or adjusted my proportioning valve because I'm not exactly sure where it is. It does make sense that it might have something to do with the vacuum boost because my pedal is waaaay too touchy like yours.

410Fortune - How do you adjust the booster plunger? Could it just be a bad one? I checked the vacuum line and there was no fluid in there. I do see some rust on the booster outer surface below the MC in a pattern like it leaked fluid and ate the paint away, but there is no visible fluid on it anywhere and the level has never dropped like it was leaking. Ideas? Thanks.

 

[410Fortune]

the front to rear biast is adjusted at the end of the plunger.
This plunger runs from the pedal, through the brake booster, and into the back of the master cylinder
You have to remove the master clyinder and adjust the tip of the plunger in or out.
Again this is set from the factory, you want to turn it in (shorter) to get more biast to teh front.

It is very touchy
If your pedal is very toughy it may be time for a new master cylinder if bleeding and purging the system doesnt fix it.