Gmt400 brakes-master cylinder upgrade, residual valves, disc/drum MC differences. So much confusion.

I've been researching this all day. From what I've read:

- The low-drag calipers, which came on all 1/2 GMT400's, require a QTU (quick take up) master cylinder.
- The QTU master works by using a large piston to send fluid to the calipers fast, then a smaller piston to build pressure
- GM realized the mistake of using low-drag calipers, so they are not used on any other generation of trucks, including GMT800
- Rear drums seem to not require the 10lb residual valve
- The leading/trailing rear drums are shit, and there's no easy way to upgrade them

So,

IF all k1500 trucks use low-drag calipers, that require a QTU master, and there were no low-drag calipers or QTU masters after 2000, is the whole master cylinder "upgrade" bullshit? Using a GMT800 master seems like it could be causing most of my problems:

- It's a disc/disc master
- It does not have a QTU feature
- Front brake pressure builds late (explained below), which could be why my drums are slow to engage and seem to barely work

The lack of the QTU feature could be the reason for the long pedal travel before my pedal firms up close to the floor; the small piston has to send enough fluid to the calipers before they start to grab the rotor because of the low-drag feature. The larger piston on QTU-equipped masters sends this fluid in a much shorter pedal stroke, so the GMT400 brake pedal initial "mush" is the QTU feature in action, sending fluid to the low-drag calipers. This mush is much longer with the GMT800 master, because of the smaller piston moving less fluid.

Is there any merit to anything I just wrote? This is just my interpretation of all of the conflicting info I've read. Makes sense to me, but I have no clue what I'm talking about, and very well may be missing something. I do find it hard to believe that everyone that does this master cylinder "upgrade" has the same issues as me and ignores them simply because their pedal feels better.

 

At the wheel end, the force pushing either the pad or shoe is dependent (for any given line pressure) on the area of the piston. The area of a calliper piston and a cylinder piston vary quite a bit. (Remember area of a circle increases with the square of the radius - doubling the diameter quadruples the area).

Admittedly a disc set-up requires more force as it has very little self servo effect (especially when the pads are worn) compared to a drum set-up but it takes a leap of faith to imagine those factors exactly compensate for the widely different forces available due to wildly different piston diameter/areas.

 

my 99 suburban had terrible squishy brakes,
i did all i could,. finally i deleted the KH abs,.
whole different truck

 

Yes. Good plan.

Use the park brake a lot. The rear drums adjust when you use and release the service brake, after using and releasing the park brake.

This is just plain negligent design. Folks go YEARS without using the park brake, especially on automatic-transmission vehicles.

Yes.

I don't know about that. Wouldn't surprise me that they were used on some Squarebodies, in the same way that the '87 Squarebodies had the same engine/transmission as the '88 GMT400s.

So far as I know, no GMT800s or newer trucks got the step-bore master cylinder. Perhaps there's some different design of low-drag caliper on them.

Yup. Likely wouldn't hurt, but shouldn't be "needed".

Upgrade rear axle to a light-duty 3/4 ton, get a stronger axle and upgraded brakes as a bonus. I did. Be sure to match the gear ratio, and use a 4x4 donor axle in a 4x4 truck, a 2wd donor axle in a 2wd truck.

Folks have a defective QTU master cylinder, they "upgrade" to a fully-functional non-QTU master from a newer vehicle, and the brakes improve. IT'S MAGIC! I DISCOVERED A MAGIC UPGRADE!. 'Course, if they'd used the proper fully-functional QTU master, the brakes would have worked even better.

Disc/disc isn't a problem...if there's no residual pressure valve designed into the disc/drum brake master cylinder. That was the only difference between a disc and a drum master cylinder aside from a potential bore-size difference...and GM quit using residual pressure valves in 1970.

Lack of the QTU feature IS a problem.

HOWEVER, I still suspect you've got air in the system, and maybe it's hiding in the rear ABS, maybe it's hiding in the master cylinder, maybe it's somewhere else. Somewhat common for air to leak INTO the drum wheel cylinders. I've also heard of folks buying aftermarket proportioning valves or Combination valves, and having air leak into the system via defective seals, without having fluid leak out.

I think you've largely got the right idea.

 

Try this. Also, do as Schurkey recommended and use the parking brake, then the service brake multiple times to help the adjustment.

 

Is that because they have different track widths?

 

Yes.

 

OK, even with "low drag" calipers the amount of fluid to squeeze them against the rotors is very small.
What you feel as "mush" is the much larger amount of fluid that has to be sent to the rear wheel cylinders.
The rears take a lot of fluid to move, and when the shoes do finally get to the drums they also shift and distort a little.
That is why the drums must be almost perfectly adjusted, with no worn parts to be "quick and firm", so to speak.
One of the main jobs of the combination valve is that it will NOT allow pressure to build-up in the front calipers until AFTER the rear shoes are expanded against the drums.

Add-on; even though their is no longer a residual valve in the MC, their is still a residual effect. Their must be for drum brakes in order to keep the cups expanded, and keep air out,, except that instead of a valve in the MC that job is now part of the combination valve.
The combination valve does a "combination" of things;
1, it operates the failure light.
2, it holds a little pressure in the rear drums.
3, it "holds back" pressure to the front discs until the rear line pressure builds up to a pre-determined level.
4, it limits rear line pressure so the rears don't go into "instant lock-up" before the fronts have a chance to "catch up", as the drums have a large servo effect and require much less pressure to operate.
Discs do not have a "little" servo effect, they have ZERO servo effect.

 

Not so.
There is moment because the friction force and the retaining forces on the pad are offset by the depth of the pad's friction material which causes the pad to be pulled toward the disc. It is slight but not zero.

 

Finally, someone who agrees. Seems like the crazy effectiveness of this "upgrade" is just solving all of the problems with a worn-out master cylinder after 200xxx miles.