Master cylinder problems 99 Suburban, HydroBoost and EVO.

[Pinger]

The only other thing that is in the back of my mind that is maybe worth mentioning.
Early days with the truck dealing with the consequences of the crankcase explosion had me move it but without the serpentine belt fitted. I had to heave on the pedal to hold the truck back with the absence of assistance. 'Heaving' is relative though as at my body weight I put half into the pedal compared to what a bigger guy might have but could I have damaged something at the pedal side? I didn't feel anything 'give' and the pedal still sits high (higher than the throttle pedal). For perspective, hauling on the steering wheel felt more savage and the rag joint wasn't troubled. Suspect I'm overthinking this part.

 

[SUBURBAN5]

Classic MC failure symptoms - and coincides exactly with GM manual's diagnosis. That, and that replacing it effects a (temporary) cure.

Well I'm no hydro boost expert lol but I'm just wondering why the hydroboost system isn't a culprit. Meaning the booster on the wall. It's just wierd to have multiple failed mc. You would think there be a lawsuit. I would not suspect the mc. Because the original one did do the same thing and 3 other mc followed the same failure. I would test the other systems specifically the booster or whatever it's called lol

 

[Dravec]

Well I'm no hydro boost expert lol but I'm just wondering why the hydroboost system isn't a culprit. Meaning the booster on the wall. It's just wierd to have multiple failed mc. You would think there be a lawsuit. I would not suspect the mc. Because the original one did do the same thing and 3 other mc followed the same failure. I would test the other systems specifically the booster or whatever it's called lol

If the booster failed, it would be nearly impossible to push on the pedal. Happened on my K3500. At 250 lbs, I'd lift myself outta my seat trying to push on it.

 

[SUBURBAN5]

If the booster failed, it would be nearly impossible to push on the pedal. Happened on my K3500. At 250 lbs, I'd lift myself outta my seat trying to push on it.

Ok makes sense I just didnt know if it could have a small enough leak to cause the pedal sink

 

[Pinger]

If the booster failed, it would be nearly impossible to push on the pedal. Happened on my K3500. At 250 lbs, I'd lift myself outta my seat trying to push on it.

That's what mine was like with the serpentine belt removed. Idling fast without the drag of the stuff driven by the serp belt I was struggling to hold it back.

If the HydroBoost is at fault, it can only be from over boosting. Is there some kind of cut-off or pressure limiter that should apply?

Noticed in the GM manual warnings not to hold 100lbs of force on the brake pedal for longer than 5 seconds with the engine running - why?
My pedal is falling with much lower pedal force that that. Probably the 25-35lbs that GM use as the test for a failed MC.

 

[Pinger]

Well I'm no hydro boost expert lol but I'm just wondering why the hydroboost system isn't a culprit. Meaning the booster on the wall. It's just wierd to have multiple failed mc. You would think there be a lawsuit. I would not suspect the mc. Because the original one did do the same thing and 3 other mc followed the same failure. I would test the other systems specifically the booster or whatever it's called lol

I'm struggling to pin it to one or the other.
From the MC perspective I can accept that the original one was worn. And that the two (identical) replacements are poor quality.
From the HB perspective I can't understand why if over-pressurising it doesn't burst a hose, force a leak at a union or slave cylinder etc.
Also, without the engine running the pedal can be forced to sinking - though their may have been residual HB assistance still in play. GM say variously 5 and 25 pedal applications to deplete it. I'm guessing 5 is nearer the mark.
EVO steering is perfect. At road speeds it's a touch heavier than I'd sometimes want - but that's not the sign of an over-boosting pump. My problem though is only detectable at slow speed (or stationary) when the steering assistance is at its highest. To my understanding, that is when the pump output is less limited.
Should HydroBoost have constant pressure (ie, like non EVO steering) or does it see varying pressure (ie, like EVO steering does)?

 

[Pinger]

Should HydroBoost have constant pressure (ie, like non EVO steering) or does it see varying pressure (ie, like EVO steering does)?

I think I have the answer to that. Looking at the plumbing the one outlet from the pump comes out beside the EVO valve and goes to the HydroBoost unit. From there (via another fitting on the HydroBoost unit - presumably a shared port) it goes to the steering, then the cooler before returning to the pump. There is also a return pipe to the pump from the HydroBoost unit (see pic).

Does anyone know what the part in the photo is for? It looks like a (female) hex head beneath a plastic (tamperproof?) plug? A control valve/orifice for the return line?

Does anyone know the flow regimes for steering and HydroBoost unit (eg, flowing when activated, flowing at all times, etc)? Should I be able to see fluid flowing into the pump reservoir from the HydroBoost unit with the engine running and neither brakes or steering being activated?

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[SUBURBAN5]

I think I have the answer to that. Looking at the plumbing the one outlet from the pump comes out beside the EVO valve and goes to the HydroBoost unit. From there (via another fitting on the HydroBoost unit - presumably a shared port) it goes to the steering, then the cooler before returning to the pump. There is also a return pipe to the pump from the HydroBoost unit (see pic).

Does anyone know what the part in the photo is for? It looks like a (female) hex head beneath a plastic (tamperproof?) plug? A control valve/orifice for the return line?

Does anyone know the flow regimes for steering and HydroBoost unit (eg, flowing when activated, flowing at all times, etc)? Should I be able to see fluid flowing into the pump reservoir from the HydroBoost unit with the engine running and neither brakes or steering being activated?

You must be registered for see images attach

Looks like s sight glass we used in ac. But I'm sure it's not lol. Wish I knew more about this system. Unfortunately mine is a c1500 with regular brake system

 

[Pinger]

Looks like s sight glass we used in ac. But I'm sure it's not lol. Wish I knew more about this system. Unfortunately mine is a c1500 with regular brake system

That's what I thought it was initially. Looks like there's a hex head in there.
I need to find out more about how EVO and Hydroboost interact. There's a huge difference in steering assistance between parking and road speeds and the Hydroboost sees the same pump output as EVO. There must be some valving that meters the Hydroboost assistance as the EVO assistance increases - for the sake of braking consistency coming to a halt. Not much in the GM manual about it though and I've not had time yet to search the 'net for info. Anything anyone knows about this would be welcomed here. The systems have been in existence for at least 25 years - surely the secrets are out by now...

 

[Pinger]

Starting to find info.
https://www.classicwinnebagos.com/forum/index.php?topic=9126.0