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

[Gibson]

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.

In order for that to happen their needs to be an angular component, either from the piston cocking in the bore, (no way,) or the caliper assembly springing/flexing, or the pad showing uneven wear features
Yes, calipers do flex on their mounts,, but for all practical purposes any servo effect is nil.
One might argue that .001 is not zero,, but for the somewhat crude disc brakes on a vehicle it might as well be.

 

[Ryan cin]

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.

Do I have a combination valve? I thought that 4WAL ABS trucks didn't use one, and all of the proportioning was handled by the abs. Also, why do the drums require so much fluid? The wheel cylinders are much smaller than the front calipers. It also seems like my brake pedal firms up from the fronts locking when the rears are barely starting to engage.

 

[Ryan cin]

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

What exactly does this do? I tried a while ago having no idea what I was doing. From what I remember the pin wouldn't push down at all, and it pushed out when the pedal was pressed.

 

[Ryan cin]

Ordered a Dorman M390259 master to try. It's supposed to be an "OEM style" replacement. I also noticed an M390257 master, which seems to be the same except with a 1.25 bore. Any benefits to the larger bore? I know larger bore=more fluid, less pressure, firmer pedal. Is the 1.25 master for hydroboost setups?

 

[Gibson]

Do I have a combination valve? I thought that 4WAL ABS trucks didn't use one, and all of the proportioning was handled by the abs. Also, why do the drums require so much fluid? The wheel cylinders are much smaller than the front calipers. It also seems like my brake pedal firms up from the fronts locking when the rears are barely starting to engage.

Even in a "low drag" caliper the pad is already in contact with the rotor.
You don't "push out the pads", as much as you just transfer pressure.
Total pad "movement" is only a few thousands of an inch, whereas in the rears the pistons in the wheel cylinders, (depending upon how well their adjusted,) move much more, that's why the MC has two different bore/piston sizes.
The big piston moves a large volume to the drums, the little piston moves very little, but at higher pressure.
Yes, on our GMT 400s their is a combination valve, it's located right next to the ABS module.
The ABS doesn't "proportion" anything, it only releases some pressure if a wheel, (or the driveshaft,) locks to allow the wheel to turn.
Their is an easy experiment you can do that will show you how far the rear shoes have to move to start any braking effort.
So, starting on a level/flat surface in gear with the parking brake released.
Don't give the vehicle any throttle, just let the rig start moving with the engine at idle,, perhaps a tiny bit to just get it moving.
At about 5mph without touching the brakes, start pushing down on the parking brake,, you'll be surprised how far you have to move the parking brake before the rig even feels like it wants to slow down.
That will give you a good idea how much the rear shoes have to move before they do anything.

 

[Gibson]

This is a crude, but easily understood example of how disc brakes work, in that they don't "push the pads out", but rely on the transfer of pressure to work;
Take a piece of 2x4, or 2x6,, it will represent the pad.
Lay it on a nice smooth surface, that will represent the rotor.
Put a couple of lbs. of weight on the board, that's the "low drag" part.
Now when you push on the board it moves easily without much drag.
Now have your 200lb. buddy stand on the board and try to push it.
The board, (pad,) didn't get any closer to the surface, (rotor,) you just added pressure.
Disc brakes work exactly the same way, they just add pressure to make more friction between two surfaces that were already touching each other.
The only "retraction" of pads is a few thousands from the piston seal.
Actual "fluid movement" from the MC is less than an eyedropper, and some of that is used for caliper flex and the bit of expansion from the rubber brake hoses.

 

[Ryan cin]

Even in a "low drag" caliper the pad is already in contact with the rotor.
You don't "push out the pads", as much as you just transfer pressure.
Total pad "movement" is only a few thousands of an inch, whereas in the rears the pistons in the wheel cylinders, (depending upon how well their adjusted,) move much more, that's why the MC has two different bore/piston sizes.
The big piston moves a large volume to the drums, the little piston moves very little, but at higher pressure.
Yes, on our GMT 400s their is a combination valve, it's located right next to the ABS module.
The ABS doesn't "proportion" anything, it only releases some pressure if a wheel, (or the driveshaft,) locks to allow the wheel to turn.
Their is an easy experiment you can do that will show you how far the rear shoes have to move to start any braking effort.
So, starting on a level/flat surface in gear with the parking brake released.
Don't give the vehicle any throttle, just let the rig start moving with the engine at idle,, perhaps a tiny bit to just get it moving.
At about 5mph without touching the brakes, start pushing down on the parking brake,, you'll be surprised how far you have to move the parking brake before the rig even feels like it wants to slow down.
That will give you a good idea how much the rear shoes have to move before they do anything.

So why is the reservoir for the rear drums smaller? In the stock QTU master if I remember right the small front part of the reservoir that holds less fluid routes to the rear. Wouldn’t it make sense that the larger reservoir would go to the rears since they require more fluid?

Thanks for all the help by the way. Appreciate it.

 

[Schurkey]

So why is the reservoir for the rear drums smaller? In the stock QTU master if I remember right the small front part of the reservoir that holds less fluid routes to the rear. Wouldn’t it make sense that the larger reservoir would go to the rears since they require more fluid?

Drum brake cylinders (Duo-Servo style) simply transfer fluid back and forth from master to wheel cylinders. All the fluid that gets pushed into the wheel cylinders to make the pistons move, goes back to the master when you let off the brakes. The only reason to have much of a reservoir at all is to prevent small leaks from depleting the fluid, and to account for temperature changes--cold fluid contracts, hot fluid expands.

Leading/Trailing shoe wheel cylinders do retain some fluid as the shoes wear and the adjuster moves the shoes closer to the drum--but the cylinders are small, they don't retain a lot of fluid from "new" shoes to "worn-out" shoes.

Disc brakes have relatively huge pistons, and as the pads wear, the pistons go further and further out of the bores. Fluid permanently transfers from master (reservoir) to the caliper bores, so you need a big reservoir. I was wrong when I said previously that the only difference between a Disc and a Drum master was the residual pressure valve--the disc reservoir is bigger, too. When the disc brake reservoir gets low, you've either got a leak, or the pads are worn-out. This is why you shouldn't add fluid to a disc brake reservoir "just because"--you destroy the pad-life indicator (low fluid level means worn pads or system leakage.)

Ideally, during pad replacement, you'd open the bleeder screws as you push the pistons back in, maybe even clamp the RUBBER brake hose (never clamp a steel-braid-over-PTFE (Teflon) hose). The old, contaminated, excess fluid leaves the system. You install fresh pads, clean the reservoir, flush the rest of the old fluid, add new fluid, verify that the brakes work. Top off the reservoir, and LEAVE IT THE HELL ALONE until the pads wear out. The vehicle owner can "see" pad wear as the progressive, slow lowering of the fluid in the reservoir.

Lots of folks just push the pistons back into the calipers, which blows a bunch of fluid back to the reservoir. If someone has "topped-off" the brake fluid, the reservoir is now over-full and dripping (or gushing) out the reservoir cap and perhaps on to painted surfaces.

Thanks for all the help by the way. Appreciate it.

[Senor Wences Voice] S'all right! [/Senor Wences Voice]

 

[Ryan cin]

Drum brake cylinders (Duo-Servo style) simply transfer fluid back and forth from master to wheel cylinders. All the fluid that gets pushed into the wheel cylinders to make the pistons move, goes back to the master when you let off the brakes. The only reason to have much of a reservoir at all is to prevent small leaks from depleting the fluid, and to account for temperature changes--cold fluid contracts, hot fluid expands.

Leading/Trailing shoe wheel cylinders do retain some fluid as the shoes wear and the adjuster moves the shoes closer to the drum--but the cylinders are small, they don't retain a lot of fluid from "new" shoes to "worn-out" shoes.

Disc brakes have relatively huge pistons, and as the pads wear, the pistons go further and further out of the bores. Fluid permanently transfers from master (reservoir) to the caliper bores, so you need a big reservoir. I was wrong when I said previously that the only difference between a Disc and a Drum master was the residual pressure valve--the disc reservoir is bigger, too. When the disc brake reservoir gets low, you've either got a leak, or the pads are worn-out. This is why you shouldn't add fluid to a disc brake reservoir "just because"--you destroy the pad-life indicator (low fluid level means worn pads or system leakage.)

Ideally, during pad replacement, you'd open the bleeder screws as you push the pistons back in, maybe even clamp the RUBBER brake hose (never clamp a steel-braid-over-PTFE (Teflon) hose). The old, contaminated, excess fluid leaves the system. You install fresh pads, clean the reservoir, flush the rest of the old fluid, add new fluid, verify that the brakes work. Top off the reservoir, and LEAVE IT THE HELL ALONE until the pads wear out. The vehicle owner can "see" pad wear as the progressive, slow lowering of the fluid in the reservoir.

Lots of folks just push the pistons back into the calipers, which blows a bunch of fluid back to the reservoir. If someone has "topped-off" the brake fluid, the reservoir is now over-full and dripping (or gushing) out the reservoir cap and perhaps on to painted surfaces.


[Senor Wences Voice] S'all right! [/Senor Wences Voice]

Interesting about the fluid level as a pad wear indicator. Never thought of it like that.

 

[Frank Enstein]

Some things that weren't mentioned. Make sure the calipers float properly. If they don't it can lead to a high/hard or low/soft pedal. Bolt the caliper down with no pads. Then push the caliper in (toward the center of the truck) and out. If you can't move it with one finger (you may have to push FIRMLY with one finger) find out why and fix it. Usually it is the rubber stuff causing the issue clean or replace it and lube it with silicone brake grease. The rear shoes dig into the backing plates and leave a flat "V" shaped groove. Grind/polish it flat, Weld/braze it up and grind it or replace the backing plates. Also "polish" the parts of the brake shoes with a wire wheel or sandpaper. You just want it smooth without making it shorter. Lube the raised pads on the backing plates with a TINY amount of moly or graphite. Coloring it with a regular No.2 pencil is fine if you made it smooth enough. With the groove when you are creeping to a stop the shoes get stuck in the groove which gives you a high hard pedal and makes the fronts do all the work. And when you stop more firmly they jump the groove and toss you out on the hood. Replace ALL the brake drum hardware. It is dirt cheap and makes them more consistent

Braided PTFE hoses are the only way to go. Makes a HUGE difference even over brand new rubber hoses.

Earl's Solo-Bleed bleed screws are the best. Pricy though.

Ceramic pads and shoes are easy on the rotor/drums, make very little brake dust, grab a bit better, and last a long time. The Powerstop Z-16 pads are great for a daily driver. Follow the pad/shoe bedding in process that the manufacturer recommends. It DOES make a difference.

Fair warning: If you do braided hoses and Solo Bleeds on one vehicle you WILL do them for everything else you own!

O.K. I'm off my soapbox now. Go have fun!