6.5 hydroboost

[phantom309]

do hydro boost units get old and weak? the 6.5 has 380,000kms,.(236,000miles)

seems that when it,s cold the rear wheels will lock kinda easy,. and when it warms up the brake pedal is really squishy,. like the brakes are full of air,. it stops sorta ok? under regular braking,. but won't slide the tires,.and i can't feel any abs pulsing,.a panic stop isn't,. just sortof rolls to a stop,.
i,m not sure if the master is weak,.? or the hydro boost is,.
anyone with experience with these things?

nick

[convert2diesel]

Good question Nick. Have a 87 and an 88, both 2,500s, both 4x4s with exactly the same symptoms. On the 88 have replaced the master and flushed the fluid, flushed the PS fluid, replaced rear brakes, front pads and flex lines and still no joy. If anything its gotten worse.

Hydro-boost appears to be functioning and have done all the tests, valves appear to be functioning (pedal drops immediately on start up). Have disconnected the ABS on both trucks (snowplows) so can't comment on this.

Would love to hear feedback on this one. A real head scratcher.

Bill

[DmaxMaverick]

The hydroboost system is much more simple than that. All it can do is amplify your pedal input. If you are having hard or spongy brakes and/or wheel/axle bias, the problem is elsewhere. Your problem sounds like an issue with air, master cylinder, isolator, or wheel cylinders/calipers. Most likely suspect is the master cylinder.

Hydroboost units do get tired and wear out. The symptoms of that happening are not what you are seeing.

[phantom309]

The hydroboost system is much more simple than that. All it can do is amplify your pedal input. If you are having hard or spongy brakes and/or wheel/axle bias, the problem is elsewhere. Your problem sounds like an issue with air, master cylinder, isolator, or wheel cylinders/calipers. Most likely suspect is the master cylinder.

Hydroboost units do get tired and wear out. The symptoms of that happening are not what you are seeing.

If i am overcoming the hydroboosts' abilty, think it will lead to a squishy pedal?
i,m ready to remove the abs and use a different master AND do a rear disc conversion,.

Nick

[DmaxMaverick]

I think you're talking to the wrong end of the horse. A rear wheel lock-up is either too much bias to the rear, or too little to the front. I suspect the latter. Underhood temps may be "helping" the system to normalize, allowing a more even brake application. If your hydroboost unit is failed or failing, it still has nothing to do with the symptoms you've described. Forget about it for now. You may have any of the following conditions: Air in the brake lines; Air-locked or failed ABS module; Water in the front lines and/or calipers; or just a worn out master cylinder.

Also, check your rear axle for a lube leak. If your rear shoes are oil soaked, it can cause rear wheels to lock, usually accompanied with a lot of vibration and chatter. Not exactly the symptoms you are seeing, but worth a look.

If you have "Rear Anti-Lock Braking system", the brake module is passive. This means it can only "unlock" the rear wheels, and operates very different than "ABS". This system has no affect on the front braking.

[rustyk]

All those should be checked, but in addition, check the P/S reservoir right after shutdown for aeration of the fluid.

[rameye]

just chiming in ,,,,same exact issues here..


Let me know if you find out whats goin on....I just assumed that the brake systems on these trucks were woefully insufficient..

New rotors/pads pins sliders on calipers...no real difference.

[DmaxMaverick]

Believe it or not.....

Almost always, it's the master cylinder. There are other possibilities of course, but people almost always insist it isn't the M/C, until they finally replace it (after throwing in a bunch of $$ on the other stuff). The M/C is one of the least expensive and easiest to replace. If it's a few years old, or of questionable quality (which applies to most of them), just replace it.

[85-m1028]

A fluid flush can't hurt "just good maintenance" I'd see if the rear drums need to be adjusted out a bit. keep an eye on the fluid level in the master, if it goes down then you just have some air in the system.

Check the fittings on the ABS unit and look for leaks.

[phantom309]

so further update,.he fluid does seem sortof bubbly after shut down,.
I tried a little idea of my own,. i drained some ps oil out,. then added some gear oil to thicken the mixture,. seems to have much better braking,.
my guess if the ps pump is low on pressure after 375,000km,.and thats why the rear brakes grab hard once or twice when its stone cold,.
rustyk you mentioned a leaking seal somewhere that creates aerated fluid?

Nick

[rustyk]

The leak I had would not apply to GM light trucks. My motorhome does have a Hydroboost system - much larger than that found on pickups.

There is no "Park" pawl on the transmission; a spring-loaded shaft brake engages when the park lever on the side of the tranny opens a valve to the actuator that releases its pressure. With the tranny in any other position (with the engine running), P/S pump pressure operates the actuator and releases the shaft brake.

This actuator was leaking internally, so that the P/S fluid was drained to the extent the pump aerated. When the engine was stopped, the entrapped leaked fluid returned - with air entrained - and overflowed the reservoir.

I agree with the consensus that the master cylinder is the chief suspect.

[spdgofast]

I have a 94 K2500 Burb and I have experienced somewhat of the same problem. I have tried to find larger high performance calipers and or rotors but all of the performance brake companies have sorta skipped this body style 4x4 brake upgrade selection. I think I will replace my MC, remove the ABS and try to find some high quality front pads with drilled and slotted rotors. Oh and by the way, my truck only had 100K when I first started experiencing this problem. I don't personally think disc brakes will stop the rear better than the 3.5 wide drum brakes on the corprate 14 bolt full floating rear axle unless they are wet and muddy the discs will self clean better but those big drums seam to not have a problem when they have good fluid pressure and are dry, just my opinion.