Bleedin' Brakes

[0xDEADBEEF]

What is it about these trucks that you can bleed all 4 corners and the pedal is hard as a rock, and then you start it up and it's mush.

Is it the ABS module?

 

[Alteca]

I think it’s because they have fairly large bores in the master cylinder so without the assist from the booster it is going to be very stiff. The large dual diagram booster give a lot of assist compared to something you would find in a car so that would dramatically change the feel when running. Same for hydroboost. I wouldn’t be surprised if abs is part of it as well but my trucks pedal still did this without abs installed.

 

[1998_K1500_Sub]

The quick-take-up piston is huge, if you've got those JB5/JB6 brakes with the QTU MC, and once the slave pistons have been "taken-up" the excess fluid needs to get shoved passed the cups of the "regular" MC pistons, at least for some duration of motion until the QTU piston is no longer in play, so there's that.

 

[0xDEADBEEF]

I've got a couple vehicles that you have to have the brake depressed to start them so it's a habit now. On my OBS trucks I can feel the pedal change as it starts.

 

[Road Trip]

I've got a couple vehicles that you have to have the brake depressed to start them so it's a habit now. On my OBS trucks I can feel the pedal change as it starts.

I've observed the same thing on my JD7-equipped chore truck. In new condition,
the Accumulator in the Hydroboost setup stores enough assist for at least 1-2
stops after the PS pump quits turning and/or before it starts turning again. (ie: Designed
to act the same as what drivers have come to expect, based upon traditional vacuum
booster behavior once the engine has been turned off.)

In not so new condition, the Accumulator in the chore truck has lost a lot (but not all)
of it's storage capability, so I am familiar with the 'rock hard up high' brake pedal that
drops down a bit once the engine fires/PS pump starts spinning. (And this is with an
otherwise well behaved brake system that's fully adjusted out back.)

This is my first Hydroboost vehicle. Acted just differently enough that I dug around
in the FSM to find out what was going on.

EDIT: Just reread your original post. I do observe the 2 different pedal heights,
pre vs post startup, but once running my pedal is still firm/confidence inspiring.

So this *is* possible in a GMT400 vehicle? In order to figure out the 'mushy w/boost'
part, the comments by the others above may account for this. Possibly @Schurkey
will weigh in, for he seems to have all the different GMT400 brake versions committed
to his mental Rolodex with indelible ink?

By the way, which flavor brakes do you have? That seems to be the key when these
brake questions come up.

 

[termite]

@Road Trip , I think he is refering to the Herd growing.

Thread 'The herd grows' https://www.gmt400.com/threads/the-herd-grows.62910/

 

[Road Trip]

Man, a full-size 2-door Blazer like that is my kryptonite! Possibly the only
vehicle that could make me feel jealous while behind the wheel of my own vehicle. :-(

494ci stroker 'Vortec 7.4' with a T-56. Sleeper edition, please.

Come on, Megabux! Hey, why don'tcha throw the old guy a bone... :0)

 

[0xDEADBEEF]

By the way, which flavor brakes do you have? That seems to be the key when these
brake questions come up.

I don't know the RPO code. Regular ol' vacuum booster and 10" drums on the back. Both the GMT400s in my "herd" have the same setup. It just seems a little excessive. I don't remember the OBS my dad or grandfather had acting that way new, but that was ~33 years ago.

The brakes are good though. I did a bunch of panic stops to bed the brake pads in and it stops way better than before.

 

[Orpedcrow]

I think it is the ABS. I had the same situation on mine a few years ago when I swapped the rearend. Didn’t have a scan tool. The gravel road lockup method seemed to help.

 

[0xDEADBEEF]

The first 2 times I did my 30-0 stops to bed the pads the right front locked up at the very end and the ABS made some whirring noises, but after that it was silent and didn't lock up. I assume it's content.

With the engine off the pedal is as solid as can be. Then you start it and it's a lot less firm. It also makes a kind of a squishy noise that isn't there when the engine is off, but maybe because it travels farther.