Bleedin' Brakes

[Alteca]

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.

Does it go hard immediately after being turned off? If so that is an issue.

 

[0xDEADBEEF]

Does it go hard immediately after being turned off? If so that is an issue.

Within a second or two.

 

[Alteca]

Within a second or two.

I’m sure someone else could explain the cause better than I could but that shouldn’t happen. You should be able to pump the brakes a few times before it goes hard, even after it’s been off for a bit.

 

[0xDEADBEEF]

I’m sure someone else could explain the cause better than I could but that shouldn’t happen. You should be able to pump the brakes a few times before it goes hard, even after it’s been off for a bit.

Yeah, it was about 2 pumps then hard.

LOL. Sounds wrong.

 

[Alteca]

Yeah, it was about 2 pumps then hard.

LOL. Sounds wrong.

One of the few scenarios where 2 pumps is acceptable

 

[Schurkey]

The quick-take-up piston is huge,

True.

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.

Not exactly.

The fluid from the bigass QTU chamber is getting pushed past the cups of the "regular" MC pistons UNTIL the caliper pistons have pushed the pads up to the rotor. Once the pads are against the rotor like a normal, "not low-drag" caliper, the fluid from the QTU chamber blows past a spring-loaded check-valve into the reservoir while the "regular" pistons/cups provide the higher PSI to clamp the pads to the rotor.


[One of many potential problems with chronic low-pedal issues on GMT400 trucks, is the need to balance the stiffness of that spring-loaded check valve against the force needed to move the caliper pistons. When the pistons get stiff (not quite seized, but harder to move than they should be) or if the check-valve spring gets weak or the check-valve leaks--all the QTU fluid is wasted by blowing into the reservoir, leaving the small "normal" piston to move enough fluid at higher pressure to get the pads up to the rotor.]

I don't know the RPO code. Regular ol' vacuum booster and 10" drums on the back.

JB/JN 3 or 5. JN3 had the weaker front brakes, smaller-bore master, and weakass power booster. Came only on regular cab trucks prior to '92, I think.

Most 1500 pickups had JB5.



Wild Guess: You need to bleed the ABS with a scan tool.
Second Guess: Power booster issues--not holding vacuum over time. Maybe as simple as a leaking check-valve in the grommet for the vacuum supply.
Third Guess: It's normal for a depleted power booster to have a hard, high pedal compared to a booster with enough vacuum to operate properly.

 

[0xDEADBEEF]

So, the kid has been driving it around town for about a month now. I drove it for the first time in a while since I posted this, and they feel perfect. I guess they just needed some miles.

 

[Caman96]

It also makes a kind of a squishy noise that isn't there when the engine is off

Possible the booster may be going with those noises.

 

[HotWheelsBurban]

Possible the booster may be going with those noises.

Yup...you may be replacing the brake booster soon.