Master cylinder problems 99 Suburban, HydroBoost and EVO.

[Pinger]

Thanks Schurkey - massive help.
The metering valve - is it upstream of the dump valves?

 

[Schurkey]

The illustration in my '97 C/K service manual shows Master cylinder to Combination Valve (including metering valve) to ABS to wheel cylinders.

So...yes.

 

[Pinger]

Thanks Schurkey. I think I have an exact diagnosis forming here. I'll give it more though and likely post it here for comments.

 

[ThaEdge]

Ok so here's what me and 2 brake techs came up with. After starting to bleed the brake system, fluid suddenly stopped coming out of the rear left bleeder (after successfully bleeding the right side). so thinking it was something upfront we opened the line up at the downstream side of the abs unit and fluid flowed freely. then it started flowing at the bleeder again. long story short it is the rear brake line causing the symptoms described in my previous post. the theory is that a part of the inside of the brake hose was moving and blocking the line causing the rear brakes to fail. it was then verified that the hose was expanding causing the excessive pedal travel. the intermittent condition we believe may have been due to the piece of loose material on the inside of the hose "flapping" back into place. don't know if this will help any one else but it solved my issue!

 

[HotWheelsBurban]

Ok so here's what me and 2 brake techs came up with. After starting to bleed the brake system, fluid suddenly stopped coming out of the rear left bleeder (after successfully bleeding the right side). so thinking it was something upfront we opened the line up at the downstream side of the abs unit and fluid flowed freely. then it started flowing at the bleeder again. long story short it is the rear brake line causing the symptoms described in my previous post. the theory is that a part of the inside of the brake hose was moving and blocking the line causing the rear brakes to fail. it was then verified that the hose was expanding causing the excessive pedal travel. the intermittent condition we believe may have been due to the piece of loose material on the inside of the hose "flapping" back into place. don't know if this will help any one else but it solved my issue!

And this is an example of why brake hoses should be replaced if they are more than a few years old ( like the original ones on a 25 year old truck).

 

[ThaEdge]

Sadly this isn't the original line. It was replaced about 2 years ago. The front are original as far as we can tell and still seem to be working fine, but we're replacing those also.

 

[Pinger]

Some progress - I think.
As the suspicion was air trapped in the ABS and I don't have a scan tool to bleed with I had to improvise (see photo).
Pushing one front wheel at a time into locking, the pedal got worse then when I bled at the caliper the pedal improved. I've now got it at a point where the pedal requires a reasonable shove to provoke the sinking (which could/would occur merely with the pedal effort required to hold at a halt on a slight hill).
I think what was happening is that the descent of the pedal was slowed by having to push fluid past a partly leaking ABS valve before the air in the ABS unit could be compressed. With that air released from the ABS unit and in the pipes, the (air) compression is immediate hence the pedal's sudden descent - until bled.

Didn't get out on the road to drive but on the driveway I'm pretty sure I'm going in the right direction here.

You must be registered for see images attach

 

[ThaEdge]

Well then...
The problem came back, kinda... Turns out I had 2 issues! Now it's working great (knock on wood). The master cylinder I purchased from the place whose name rhymes with auto zone was faulty from the get go! Had a small peice of rubber floating up and clogging the port fully! So that feeling of not having rear brakes was an accurate premonition. Peddle is firm and brakes apply smoothly and firmly at every application now! Hopefully this is the end of this for me! Good luck everone!

 

[Pinger]

Bit of an update on this - more of a rolling blog...
Since flushing the PAS and hence HydroBoost and the bleeding I got a proper chance to drive it (325 miles over the twists and hills of Highland Scotland) and the brakes are definitely different than before.
I haven't totally eradicated the sinking pedal effect but it has to be provoked now (and I didn't provoke it) but at least I'm not in constant fear of the pedal heading for the floor (though even when it did, there was always enough stopping power).
It feels much less 'assisted' now - feels like my foot is doing more of the work as opposed to the pedal being powered away from under my foot. I have to push on the pedal now and it feels more like brakes should. The initial 'bite' - that felt like it was generated by my foot opening a valve and letting the HB do all the work has gone. Whether this is down to fresh fluid in the HB or the bleeding I don't know. The pedal is still soft (to begin with) and springy so I think there's more air in the lines to bleed out. The passenger side front brake seems to be coming in before the driver side one - suggesting air in the latter.
More work to do but definitely going in the right direction.

 

[SUBURBAN5]

Bit of an update on this - more of a rolling blog...
Since flushing the PAS and hence HydroBoost and the bleeding I got a proper chance to drive it (325 miles over the twists and hills of Highland Scotland) and the brakes are definitely different than before.
I haven't totally eradicated the sinking pedal effect but it has to be provoked now (and I didn't provoke it) but at least I'm not in constant fear of the pedal heading for the floor (though even when it did, there was always enough stopping power).
It feels much less 'assisted' now - feels like my foot is doing more of the work as opposed to the pedal being powered away from under my foot. I have to push on the pedal now and it feels more like brakes should. The initial 'bite' - that felt like it was generated by my foot opening a valve and letting the HB do all the work has gone. Whether this is down to fresh fluid in the HB or the bleeding I don't know. The pedal is still soft (to begin with) and springy so I think there's more air in the lines to bleed out. The passenger side front brake seems to be coming in before the driver side one - suggesting air in the latter.
More work to do but definitely going in the right direction.

At least progress.. awesome job. Now you'll be a bleeding brake expert