Braking trouble

[AuroraGirl]

brakes are all new at the back(they are drums, right?) that to me sounds like air, residual pressure valve, kelsey-hayes RABS stuff, adjustment, parking brake cable adjustments, proportioning valve if in this system, proper spring attachments... etc...

 

[Pinger]

The compressor sound does imply the ABS motor kicking in but normally when ABS is invoked the pedal is going away from you and pulsating giving a soft feeling as a result. Not sure about GMT400s but pretty certain that my last car lit up a dash light when ABS was operating. If it is an ABS issue (remember that ABS momentarily releases the brakes so loss of braking is a compatible symptom) check out the comments in post #9 re reluctor rings.

What I sometimes experience with my Suburban with HydraBoost is when the engine is cold and idling very very slowly, and I'm shunting to get off the driveway, I'll sometimes feel the pedal pulsate and go hard. Usually, this is followed by the engine stalling. Never a problem when warmed up, only when cold when it idles absurdly slowly.

 

[AuroraGirl]

The compressor sound does imply the ABS motor kicking in but normally when ABS is invoked the pedal is going away from you and pulsating giving a soft feeling as a result. Not sure about GMT400s but pretty certain that my last car lit up a dash light when ABS was operating. If it is an ABS issue (remember that ABS momentarily releases the brakes so loss of braking is a compatible symptom) check out the comments in post #9 re reluctor rings.

What I sometimes experience with my Suburban with HydraBoost is when the engine is cold and idling very very slowly, and I'm shunting to get off the driveway, I'll sometimes feel the pedal pulsate and go hard. Usually, this is followed by the engine stalling. Never a problem when warmed up, only when cold when it idles absurdly slowly.

i would clean your IAC, passages, and tb, and set the minimum throttle if out of spec. unless your suburban uses a switch the ECM sees as input when the engine is being loaded on the ps or the hydroboost. I would also verify that the hydroboost isnt have bad internal seals or something allowing pressure to bleed off cold and loading the pump to replenish, same with steering. check steering geometry pivots and grease, What ps pump, what fluid, any cooler piped in the system, what climate you live in, hows your tensioner

and finally, how is your alternator and battery
what kind of alternator is it?

 

[Dariusz Salomon]

You could just say what exactly happens and if any maintenance was done before or after the problem you are experiencing.

I did explain the best I can what exactly is happening. As to maintenance-nothing related to braking system recently-I said rear brakes are new but I consider new as less than a year ago. When I was thinking about the problem the best to describe what may be going on is the booster is working as intended until the very last moment when either the valve lets in a bit of air or booster beggins to develop a leak and losses the pressure.
It seem to be getting worse hence I'm thinking something's problem is expanding-like idk-crack in vacuum pipe getting bigger etc. This is just a theory to describe symptoms the best I can-just a theory and it may not be the reason.

 

[Dariusz Salomon]

Ive dealt with that same issue and mine was from the reluctor wheel and abs sensor with build up crud on back side of rotor would only do it at slow speed stopping.

On the scan tool you could watch the wheel speed sensor signal flat line and abs would activate.

That is a great suggestion as it describes exactly what I'm dealing with.

 

[Dariusz Salomon]

getaway sounds like while braking the assist is either lost or the rear brakes arent adjusted right, and then the truck starts trying to push the truck while it downshifts to 1st and the idle speed is decent. Would be hell of a noticeable event in the snow and ice probably start fishtailing while barely moving

Luckily we don't have much snow here(if any) but I was also thinking if the moment of downshift has anything to do with it.

 

[Pinger]

i would clean your IAC, passages, and tb, and set the minimum throttle if out of spec. unless your suburban uses a switch the ECM sees as input when the engine is being loaded on the ps or the hydroboost. I would also verify that the hydroboost isnt have bad internal seals or something allowing pressure to bleed off cold and loading the pump to replenish, same with steering. check steering geometry pivots and grease, What ps pump, what fluid, any cooler piped in the system, what climate you live in, hows your tensioner

and finally, how is your alternator and battery
what kind of alternator is it?

It's just a quirk I live with that I can eradicate easily by adjusting the butterfly to a more open position but if I do that it idles faster than necessary when warm so I tolerate it when cold. As you've likely guessed the butterfly adjustment has been messed with probably during the LPG (propane) system install. My setting and the faster hot idle speed if opened is probably a consequence of running on LPG. All the other stuff you mention has been checked and up to *****.

 

[Erik the Awful]

My son's first Crown Vic would activate the ABS at almost every stop. We fixed one of the wheel sensors, but something else was wrong and he fubar'd the engine before we got it sorted.

I'd check the reluctor wheels and pickups. Make sure they're all clean, and check the gap between the pickups and reluctors. Check the condition of the wiring as well.

 

[Dariusz Salomon]

My son's first Crown Vic would activate the ABS at almost every stop. We fixed one of the wheel sensors, but something else was wrong and he fubar'd the engine before we got it sorted.

I'd check the reluctor wheels and pickups. Make sure they're all clean, and check the gap between the pickups and reluctors. Check the condition of the wiring as well.

I must say that I didn't think about ABS sensor as a possible culprit at all but now it actually make hell of a lot of sense. What I was assuming was losing a braking boost may in fact be abs kickin in with similar sensation-it is precisely why I posted this question here cause sometimes obvious is not the solution but completly different approach may be needed. Thank you all for your input-I appreciate your help and as soon as I have time to address the issue I will post my findings. That doesn't mean I close the disscusion so if anybody has anything to add feel free to post. Merry Xmas everyone

 

[AuroraGirl]

It's just a quirk I live with that I can eradicate easily by adjusting the butterfly to a more open position but if I do that it idles faster than necessary when warm so I tolerate it when cold. As you've likely guessed the butterfly adjustment has been messed with probably during the LPG (propane) system install. My setting and the faster hot idle speed if opened is probably a consequence of running on LPG. All the other stuff you mention has been checked and up to *****.

"The spacer you need may already be on the vehicle to raise your OEM aircleaner. If so the short adaptor will work fine. Be very careful tigtening down the spacer though. The spokes on this spacer can be easily broken if you overtighten the holddown nut. Also since this nut is under the mixer use locktite or some silcone on the nut. If it come loose for any reason the nut could end up inside the engine. Lastly, if you use the OEM spacer make sure you relocate the PCV breather to the new aircleaner or use an external filter. If you leave it in its factory location it will be greathing under the mixer and sucking propane vapour through the crank case."

That may be something to consider if you didnt already handle it. But nice!