Master cylinder problems 99 Suburban, HydroBoost and EVO.

Schurkey

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Take one of your "failed" master cylinders apart, look for defects. MAYBE you find something "odd" on one or more of the seals. Or damage in the bore.

Sample master cylinder--yours won't have the bigass rear piston/seal. Should be the same size as the other seals.
http://hbassociates.us/K1500_Front_Suspension_54.jpg
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Pretty much all "new" master cylinders come from Communist China. Very likely that all master cylinder rebuild kits are sourced from the same folks as make the "new" master cylinders. Shameful!

As is so often the case, "intermittent" problems tend to be the hardest to fix. And your brake pedal only drops "sometimes".

I have a hard time believing that the hydroboost is to blame. Even if it was over-boosting the master cylinder, I'd expect brake hose failure before master cylinder seal failure.

I think this is in the ABS system--there's various valves that block or allow fluid flow, and accumulator(s) that store pressure. A failure in the valve/accumulator/pump system could cause a loss of brake pedal height. I find it curious that this tends to happen at the same time that the wheel speed sensors are generating minimum signal voltage/frequency.

'Course, that brings us back to having a scan tool that can communicate with the ABS.
 
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Pinger

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Take one of your "failed" master cylinders apart, look for defects. MAYBE you find something "odd" on one or more of the seals. Or damage in the bore.

Sample master cylinder--yours won't have the bigass rear piston/seal. Should be the same size as the other seals.
http://hbassociates.us/K1500_Front_Suspension_54.jpg
You must be registered for see images attach


Pretty much all "new" master cylinders come from Communist China. Very likely that all master cylinder rebuild kits are sourced from the same folks as make the "new" master cylinders. Shameful!

As soon as I get time I intend stripping the original MC. If the Rock Auto replacement isn't going back as warranty claim, it too.
To look for damage and, get seal dimensions in the hope that I can source better quality seals directly from the likes of Lockheed, Girling, ATE, et al.

As is so often the case, "intermittent" problems tend to be the hardest to fix. And your brake pedal only drops "sometimes".

The slow descent of the pedal when stationary is a constant. The sudden descent to the floor unpredictable but when it has happened (twice - the truck's original MC and the second Rock Auto replacement) it has been removed from the truck. There is no recovery from the sudden descent scenario.

I have a hard time believing that the hydroboost is to blame. Even if it was over-boosting the master cylinder, I'd expect brake hose failure before master cylinder seal failure.

Inclined to agree. If it is, then the return fluid line is suspect (easily checked) along with a sticking spool valve but the consistent and predictable application of the brakes makes that unlikely - unless sticky only in the return direction. That the pedal will also sink without the engine running (and reserve assistance depleted) also points to HB not being the problem. And, although I'm highlighting the problem occurring at low speed when the EVO provides maximum pressure - non EVO trucks run at that pressure all the time without problem.

I think this is in the ABS system
--there's various valves that block or allow fluid flow, and accumulator(s) that store pressure. A failure in the valve/accumulator/pump system could cause a loss of brake pedal height. I find it curious that this tends to happen at the same time that the wheel speed sensors are generating minimum signal voltage/frequency.

My smart also at times exhibited the slow descent of the pedal when stationary - but to a lesser extent and that it did returns to my mind as being something brake systems with ABS can do. Never a complete plunge to the floor though. The mild sinking of the pedal isn't in itself a problem I can't live with (and pretty sure it was present when the truck passed the annual safety inspection with the tester operating the brakes on test rollers). It is the tendency for the sudden sinking that is worrying.
Early days with the truck I did some front brake work - mainly checking that the caliper pistons were free. I can't for the life of me remember if I opened the bleed valve before pushing the pistons into the caliper. There is the nagging doubt that I pushed some dirt into the ABS unit.
But, subsequently (on a slippery surface) I have provoked the ABS into operation enough I'd have thought to push fluid through and any debris with it. Also, while the slow descent of the pedal is compatible with a slightly leaking valve the sudden descent isn't. Especially as when it has occurred it has been well away from any possibility of ABS intervention.

'Course, that brings us back to having a scan tool that can communicate with the ABS.

Quite so!

Thanks Shurkey, you've helped with the next lines of enquiry.
A couple of checks to eliminate HB as culprit.
Examine 'failed' MCs for damage.
Consider ABS and how it might be the cause - including - should the pedal suddenly plunge to the floor with the currently installed MC - the possibility of rectifying without immediate replacing of the MC. Though replacing the MC has always effected a cure when even the day following the complete plunge to the floor scenario the problem persists (ie, not self rectifying). Replacing the MC obviously involves bleeding so not just MC replacement in isolation.
In that scenario though - and the most recent occurrence of it felt just like a when a brake pipe fails - there was strong braking at the very end of the pedal travel. As though one of the brake circuits is being 'lost'.
 
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Pinger

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As soon as I get time I intend stripping the original MC.

Got that done and a quick perusal shows no damage. Wear possibly/probably but not tears or deformations.

Re the ABS unit - is there anything (physically) to be looking at externally? What about the pin things that reside under rubber caps? Is one for resetting a proportioning valve?
 

HotWheelsBurban

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Y'all please pardon me for asking what may be a dumb question, but do the 400 trucks still have a brake proportioning valve? Haven't been under my Burb in a while, so I don't remember seeing one like my square body trucks had.
Although none of them were hydro boost. One thing to remember about the OP's Burb is that it's low mileage, so it's been parked a high percentage of its life....mostly in cold damp climate. That would be rough on brake systems, evident by he's had to replace the brake hard lines already.
 

Pinger

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Y'all please pardon me for asking what may be a dumb question, but do the 400 trucks still have a brake proportioning valve? Haven't been under my Burb in a while, so I don't remember seeing one like my square body trucks had.

I think there is one within the ABS unit. I seem to recall someone saying the pin behind the rubber cap that points forwards is for resetting it. Can't be sure though and someone will know for sure.
(One reason to start this thread was to try and bring all the fragments of knowledge together in one place that relate to brake problems other than the usual at-the-wheel stuff).


Although none of them were hydro boost. One thing to remember about the OP's Burb is that it's low mileage, so it's been parked a high percentage of its life....mostly in cold damp climate. That would be rough on brake systems, evident by he's had to replace the brake hard lines already.

It's not so very bad! I think it was garaged much of its life and it's only in the last few years that it's been this far north. England has a much kinder climate. That a hardline lasted 22 years is a novelty. Ten years in NE Scotland will finish a steel line.
 

SUBURBAN5

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Pinger right. The proportioning valve is within the abs unit.
Now as far as looking for issues I would look at the pins. Mine actually leak a little when I brake hard or mess around. But there not constantly leaking. All I know is if you trap air your almost sol and you'll have to have the dealer work on it. Depending how you bled the brakes it's very easy to trap air. Knock on wood I'm gonna test my brakes once the fluid gets here and I can flush the brakes 1 last time. Before making sure there working normal.
Now since you've changed your mc multiple times I gotta ask how did you bleed the brakes from mc to abs module?
 

Pinger

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Pinger right. The proportioning valve is within the abs unit.

Just read the same on another thread.

Now as far as looking for issues I would look at the pins. Mine actually leak a little when I brake hard or mess around. But there not constantly leaking. All I know is if you trap air your almost sol and you'll have to have the dealer work on it. Depending how you bled the brakes it's very easy to trap air. Knock on wood I'm gonna test my brakes once the fluid gets here and I can flush the brakes 1 last time. Before making sure there working normal.

If they are leaking outwards - can they draw air inwards?

I'm regretting rushing to change the MC this last time. Beginning to wonder if what I experienced wasn't air being expelled from the ABS unit that could have been bled out - or something similar. The pedal going to the floor was as sudden and lacking resistance as to feel like the loss of one circuit.
Now it's back together and working (other than the slow sinking) any clues are lost.


Now since you've changed your mc multiple times I gotta ask how did you bleed the brakes from mc to abs module?

At the MC pipe unions.
At the wheel bleeding also but rarely any air there. There was a lot this last time though at the driver side wheel. It did sit overnight between MC being removed and refitted. Normally that is done as rapidly as possible.
I have difficulty with the concept of pushing a bubble (of air) downwards. Evidently, that is what I did.
 
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