Master cylinder problems 99 Suburban, HydroBoost and EVO.

[SUBURBAN5]

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[SUBURBAN5]

Heres another one may be better

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[Pinger]

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I did that ages ago and got a spurt of fluid. Any time since, nothing - unless that was air, Not done it in while though.

But, it should bleed the ABS unit - yes?

 

[SUBURBAN5]

I did that ages ago and got a spurt of fluid. Any time since, nothing - unless that was air, Not done it in while though.

But, it should bleed the ABS unit - yes?

It should accept idk what happens if the abs traps air. For the past 23 yrs we had the dealer do brakes. This year the first I replaced calipers, lines, and rear wheel cylinders by myself. That being said I bled all my lines from the rear to front but still want to bleed it to ensure theres no air trapped. Also I did not replace the mc, that was the dealer 3 yrs ago and i remember him telling me he bleed it several times to get the air out but it was a pain

 

[Schurkey]

Scan tools have an ABS bleed function for these ABS units for a reason. That's the REAL procedure.

Anything else is just a band-aid. A work-around for the proper procedure.

 

[Pinger]

Bled at the thing on the side of the ABS unit and at the unions again and no better - possibly worse.
Away to re-read the HydroBoost test in the GM manual as from memory what is expected is a slow descent of the pedal as the engine fires then a progressive push back. Mine drops very slightly quickly then kicks back with two harsh and noisy kicks.

Had the cap off the PAS pump reservoir and couldn't see any fluid returning from the HB unit - or disturbance in the fluid from it. The fluid is so still it gives a perfect mirrored reflection to the point where I thought I was looking at a polished surface but when I dip the cap in it it shows to the cold level.

 

[ThaEdge]

Ok, so I have not really delved into this but here is my issue:


1996 C3500 Hydroboost. Similarly, to Pinger, I had an issue with the MC to start with. It started the same with the pedal sinking to the floor slowly when stopping but mine progressively got worse over the course of a week or so. I replaced it with an O'Reilly gold. Bleeding this thing was an absolute nightmare!!! My system for doing a bench bleed is installing the MC but not connecting the brake lines. Then I get a couple of brake lines that will connect to the ports on the MC and cut them off about 2 inches long. then I use a clear silicone tubing to connect to it and run it back into the reservoir. then I start pumping the pedal. the fluid circulates and eventually you get all the air out. not this time! the front cylinder blead just fine this way but not the back. it would not actually circulate the fluid. it would just push and pull from the pressure side. so I fashioned a small mason jar with a lid and a couple of fittings to push fluid back into the cylinder from the outlet. this seemed to have worked. then I reconnected the lines back to the MC making sure to have a slight bend to ensure air bubbles stayed at the top (I did not want to deal with air in the ABS unit). I then got my girlfriend to help with the pedal and proceeded to bleed the air out at the fittings. now you're all caught up.


The pedal is firm. The brakes work great, sometimes. The rest of the time I have verry little back brakes. Also there is a lot of pedal travel intermittently (almost to the floor). I had originally thought it was air still in the system. But if that’s the case why is the pedal so firm? And why is it intermittent? Also, the brakes never work right first thing in the morning or after the truck has been sitting for a few hours. Also, if I press the pedal slowly there is a major increase in brake performance. If I press the pedal quickly, like in the case of an emergency braking situation, I get a slight kick back in the pedal and there are no back brakes at all it would seem. Also during this type of situation, the pedal will go almost to the floor before the brakes begin to engage. Pumping the brakes helps temporarily. Sometimes the brake light comes on. When this happens and I’m towing 10,000lbs it’s a real problem as the back brakes are needed much more when towing. it will just lock up the front brakes, the abs will cycle and it will basically coast to a stop. I have not checked the rear brake adjusters yet. I hope to have time this weekend as I’m going to need to have her ready to work next week, I have a couple of skid steer jobs and my old dodge just doesn’t like towing that much weight.


To recap;


Brake pedal is always firm even when it has a long travel.

Back brake performance coincides with the pedal travel (i.e. with a long travel the back brakes appear to not work).

The pedal always has too much travel (over an inch) even when appearing to function correctly.

Pumping the brakes for 2 to 3 minutes will alleviate the issue temporarily. The time that this lasts varies widely from several hours to less than 5 minutes.

There are no wet lines or wheel cylinders, indicating leaks.

I am quite confident that I didn’t get any air in the abs unit, so I don’t believe that to be an issue.

any input would be appreciated!

 

[Schurkey]

First Guess: Problems with, or air inside the dump valve for rear brakes in the ABS unit.

Proper bleeding MAY eliminate the issue. You'll need a gallon + of brake fluid and a scan tool, if you're using the "proper" procedure.

 

[Pinger]

First Guess: Problems with, or air inside the dump valve for rear brakes in the ABS unit.

Proper bleeding MAY eliminate the issue. You'll need a gallon + of brake fluid and a scan tool, if you're using the "proper" procedure.

From the attached document in the 'low pressure' method: ''
2. Install tool J 39177 to press the metering stem of the combination valve mounted on the BPMV assembly.
''

Is the metering stem the pokey out (when the pedal is depressed) thing on the driver side of the BPMV assembly and what is the significance of it?

Still can't work out how air can enter the ABS unit during bleeding operations (or any other circumstance) without the ABS actually being activated. If its valves are closed (unless being activated) how can air enter? Leaking valves? If so, does bleeding effect a cure to the problems here and any leaks in the valves are of no importance?

 

[Schurkey]

From the attached document in the 'low pressure' method: ''
2. Install tool J 39177 to press the metering stem of the combination valve mounted on the BPMV assembly.
''

Is the metering stem the pokey out (when the pedal is depressed) thing on the driver side of the BPMV assembly and what is the significance of it?

A metering valve blocks fluid pressure to the front brakes until brake pressure is high enough to move the rear shoes against return-spring tension. This allows the front and rear brakes to apply at the same time, instead of the front brakes applying before the rears.

The metering valve is a "three position" sort of deal.
1. At rest (zero or extremely low pressure--let's say "less than 20 psi) the metering valve "vents" to allow slight fluid flow between front calipers and master cylinder. This allows fluid to expand and contract with changes in temperature, so it can not build pressure or vacuum in the hydraulic system.

2. With low pressure, (let's say ~20--100 psi) the valve closes, blocking pressure to the front brakes.

3. At higher pressure--for example, above 100 psi--the valve is "blown open" so the front brakes get pressure and apply.

The problem is that a pressure bleeder "can" apply enough pressure to cause the metering valve to close, blocking fluid flow to the front brakes. This makes bleeding the front brake impossible.

The "special tool" forces the metering valve into the vent position to allow fluid flow.

"I" use very low pressure (10--15 psi) in the pressure bleeder, so that the metering valve is still in the "vent" position, allowing fluid flow. I have NEVER needed to block the metering valve to bleed front brakes. Having said that, I've also not bleed many of these ABS systems--so MAYBE blocking the valve with the special tool on these vehicles is actually helpful. The way I see it--if the front brakes are actually bleeding, you didn't need the "special tool". If you can't get fluid flow in front, you do (or the bleeder screw is plugged, or the brake hose(s) are plugged, or the metal tubing is crimped shut somewhere, or the isolation valve(s) are seized closed.)

Still can't work out how air can enter the ABS unit during bleeding operations (or any other circumstance) without the ABS actually being activated. If its valves are closed (unless being activated) how can air enter? Leaking valves? If so, does bleeding effect a cure to the problems here and any leaks in the valves are of no importance?

Leaking valves is my first guess at the actual problem. Air in the system for whatever reason, goes past the leaking dump valve into the accumulator and becomes trapped. Applying the brakes ever after also allows fluid pressure into the accumulator via the leaking dump valve--and both the air and the accumulator spring compresses under pressure leading to a low pedal. There's three dump valves controlling two accumulators, and any/all of them could leak or have air.

MAYBE cycling the dump valve(s) will knock a rust particle off of the valve/seat, so that the dump valve doesn't leak any more; and cycling the system should vent the air out of the accumulator, so it can be bled out. OTOH, the dump valve could be seized open, in which case you're screwed.

I notice that they expect you to "bleed" a GALLON of fluid during this process--half a gallon to start, cycle the ABS four times, then bleed another half-gallon. I suspect this is a "worst case" deal, you're told to bleed a large amount of fluid to assure you've moved all possible air out.

You go out on your gravel road, and "activate the ABS", which supposedly works like the scan tool to cycle the valves. But unless you're activating ALL THREE CHANNELS (LF, RF, Rear) you're not cycling all three dump valves. More likely, you're repeatedly activating the same dump valve, while leaving one or two dump valves inactive. The "gravel road" method might work. Or it might not.