'89 C2500 rear brakes lock up

[ASJT3]

Hey fellas,

I just picked up a C2500 with the 350 and 250,000 miles on it. Looks like the brakes were redone in the past few years.

The problem is, whenever you stomp on the brakes, the rears lock up immediately. It will spin out easily in the rain and squeal like hell on dry pavement, so slow controlled stops are necessary.

I'm not sure what could be causing this. Before I go tearing apart the system, does this sound familiair to anybody? Whoever replaced lines to the MC didn't coil them underneath the MC, is that an issue?

Thanks and sorry for the noob question.

-Alex

[gmtech1]

First thing I would check would be if the axle seals are leaking onto the brake shoes. No the coiling will not affect the braking.

[j cAT]

Originally Posted by ASJT3

Hey fellas,

I just picked up a C2500 with the 350 and 250,000 miles on it. Looks like the brakes were redone in the past few years.

The problem is, whenever you stomp on the brakes, the rears lock up immediately. It will spin out easily in the rain and squeal like hell on dry pavement, so slow controlled stops are necessary.

I'm not sure what could be causing this. Before I go tearing apart the system, does this sound familiair to anybody? Whoever replaced lines to the MC didn't coil them underneath the MC, is that an issue?

Thanks and sorry for the noob question.

-Alex

could be the rear axle seals leaking oil on the drums. also check rear wheel bearings..

[Schurkey]

Originally Posted by j cAT

could be the rear axle seals leaking oil on the drums. also check rear wheel bearings..

Or the wheel cylinders leaking brake fluid on the shoes/drums. Any contamination of the shoes will make for grabby brakes.

While you're in there, assure that the primary shoe is forward, and the secondary shoe is to the rear. Verify that the star-wheel adjuster isn't damaged or seized--and that the shoes are therefore properly adjusted.

If the bearings are defective, the axle shafts will be defective, too. The bearings ride directly on the axle shafts. Thanks, GM. Wonderful engineering.

[MagicRat]

Originally Posted by Schurkey

If the bearings are defective, the axle shafts will be defective, too. The bearings ride directly on the axle shafts. Thanks, GM. Wonderful engineering.

To be fair to GM, many manufacturers did this on live axles for years. I recall its a "semi-floating axle"

One can get offset replacement bearings that ride on the remaining undamaged part of the axle shaft.

[Schurkey]

Originally Posted by MagicRat

To be fair to GM, many manufacturers did this on live axles for years. I recall its a "semi-floating axle"

It is a semi-floating axle, but having the bearings ride directly on the shaft is not a defining characteristic of semi-floating. Other semi-floating axles have inner races for the bearings so they don't use the axle shaft as a wear surface.

Semi-floating just means the axle shaft is load-bearing at the outer end, as opposed to a full-floating axle where the hub carries the weight and the axle shaft does nothing but provide torque to the hub.

Originally Posted by MagicRat

One can get offset replacement bearings that ride on the remaining undamaged part of the axle shaft.

Yes, I have a set of those on my El Camino. The downside is that the grooved, worn part of the axle shaft becomes a stress-riser. If it's bad enough, the axle could eventually crack there. Fine for Granny, not so great for a performance application.

[j cAT]

Originally Posted by Schurkey

It is a semi-floating axle, but having the bearings ride directly on the shaft is not a defining characteristic of semi-floating. Other semi-floating axles have inner races for the bearings so they don't use the axle shaft as a wear surface.

Semi-floating just means the axle shaft is load-bearing at the outer end, as opposed to a full-floating axle where the hub carries the weight and the axle shaft does nothing but provide torque to the hub.


Yes, I have a set of those on my El Camino. The downside is that the grooved, worn part of the axle shaft becomes a stress-riser. If it's bad enough, the axle could eventually crack there. Fine for Granny, not so great for a performance application.

the axle breaking off at this point I have knowledge of... with these type bearing set ups. it did happen in these older vehicles. one thing about a bearing failure would be a bent axle . these axles do bend easy . so if the wheel hit or got hit then the weak spot is this bearing area and with the bearing getting hot then cold creates a very nice break point.

for the most part this type set up works for several hundred K miles ,but this is with the axle not being stressed out... like easy driving. hard driving bad road will kill this type design .. GM did a poor job with this ...

[ASJT3]

Originally Posted by j cAT

could be the rear axle seals leaking oil on the drums. also check rear wheel bearings..

I checked the rears, clean as a whistle. Lots of pad left too. I'm thinking there's too much power to the rear brakes. could this this be a valve block issue?

[Schurkey]

Originally Posted by ASJT3

I'm thinking there's too much power to the rear brakes. could this this be a valve block issue?

Extremely unlikely. Do the shoes seat against the anchor pin at the top--or is one of them held away from the anchor pin?

[777stickman]

Originally Posted by ASJT3

I checked the rears, clean as a whistle. Lots of pad left too. I'm thinking there's too much power to the rear brakes. could this this be a valve block issue?

I've been following your post and was thinking a bad Proportioning Valve also. Could also be a bad M/C? I've had that happen in the past!! To test this you would need to disconnect and plug the outlet to the rears at the M/C or P/valve to see what kind of braking you have to the fronts.

Does the "RED" brake light come on when cranking the motor?

[Schurkey]

A proportioning valve does not activate until the pressure is quite high. Therefore, no proportioning valve action at typical mild- to medium-braking.

There's nothing in the master cylinder that can provide excess pressure to the rear brakes.

Might be that the front brakes are somehow weak, leading to over-use of the rears.

[j cAT]

Originally Posted by ASJT3

I checked the rears, clean as a whistle. Lots of pad left too. I'm thinking there's too much power to the rear brakes. could this this be a valve block issue?

I also agree that the brake valving will not cause your issue .. as was mention though the front brakes may not be doing the proper brake effort , so the rears are doing most of the braking... this may mean the brake hoses are bad to the front calipers, or the many other reasons that the front brake pads are not being applied at the proper force to balance the vehicles front to rear brake effort..........

with bleeding the front brakes , this should show , open the bleed screw place hose on the fitting and have a helper hit the brakes see how the brake fluid flows...

with bleeder screw open use C clamp push back caliper see what the brake fluid looks like dirty/air/discolored >>>>.

always use a clear tubing so you can see what the brake fluid looks like coming out the bleeder screw...

[ASJT3]

Originally Posted by j cAT

I also agree that the brake valving will not cause your issue .. as was mention though the front brakes may not be doing the proper brake effort , so the rears are doing most of the braking... this may mean the brake hoses are bad to the front calipers, or the many other reasons that the front brake pads are not being applied at the proper force to balance the vehicles front to rear brake effort..........

with bleeding the front brakes , this should show , open the bleed screw place hose on the fitting and have a helper hit the brakes see how the brake fluid flows...

with bleeder screw open use C clamp push back caliper see what the brake fluid looks like dirty/air/discolored >>>>.

always use a clear tubing so you can see what the brake fluid looks like coming out the bleeder screw...

I'll check on the function of the front brakes and report back.

[777stickman]

Originally Posted by Schurkey

A proportioning valve does not activate until the pressure is quite high. Therefore, no proportioning valve action at typical mild- to medium-braking.

Maybe the shuttle on the P/Valve, for some reason has blocked off the fronts??

Originally Posted by Schurkey

There's nothing in the master cylinder that can provide excess pressure to the rear brakes.

But there are two tanks for front & rear on the M/C and one piston. If the front seal is bypassing it could lead to less press on the fronts and more to the backs.

Which leads into your last quote

Originally Posted by Schurkey

Might be that the front brakes are somehow weak, leading to over-use of the rears.

[Schurkey]

Originally Posted by 777stickman

Maybe the shuttle on the P/Valve, for some reason has blocked off the fronts??

Nowhere in the factory literature OR in my own experience does the safety valve "block" one or the other hydraulic circuits when it's moved off-center. OTOH, I hear about it blocking a circuit from forum posters fairly often...so maybe there's something to this after all.

If "P/Valve" is short for "proportioning valve", keep in mind that the proportioning valve has NOTHING to do with front brake operation, which is controlled in part by the metering (holdoff) valve. The safety switch is between the two (front and rear) hydraulic circuits.

Originally Posted by 777stickman

But there are two tanks for front & rear on the M/C and one piston.

There is one actual cylinder, with two pistons end-to end--the primary piston is moved directly by the brake pedal pushrod, or by the power booster pushrod. The secondary piston normally moves in relation to the fluid pressure created by the primary piston, but if there's a hydraulic failure in the primary circuit, the primary piston will move the secondary mechanically.

If there's a failure in the secondary circuit, the primary circuit can still generate pressure when the secondary piston bottoms-out.

Originally Posted by 777stickman

If the front seal is bypassing it could lead to less press on the fronts and more to the backs.

Whichever circuit fails, creates additional pedal travel to get the other circuit to generate pressure. Having one hydraulic circuit fail cannot induce extra pressure into the other, but the brake pedal will probably be pressed harder to get two brakes to do the work of four; especially if it's the front brakes that fail.