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

[arrg]

Y’all got me worried this is my issue. I went with a M630283 MC off amazon, when I look it up on summit I believe it says It’s for 4 disc can anyone confirm this? If it is can someone give me a part number for a disk/drum MC

The point I was trying to make above, it that your master cylinder choice is likely not the reason your rear drums aren't working. Go adjust your brakes and bleed them before you shoot the parts cannon at it again.

 

[Knuckle Dragger]

Short answer is: If you have rear drums, then yes. By swapping over to a disc/disc MC you've effectively made them 80% useless when you step on your brake pedal.

In a disc/disc MC, the residual pressure in the lines is about 2 psi front and 2 psi going to the rear brakes vs. 2 psi front and 10 psi rear pressure in a disc/drum MC. Due to the pressure difference, the rear brakes cannot engage as quick since the brake line pressure needs to build to 10 psi before the rear drums start to engage. This results in increased brake pedal travel before the rear brakes actuate (i.e. longer stopping distance, and a spongy pedal feel) If you want to fix your braking setup I would either swap out the disc/disc MC back to a disc/drum MC, or install a residual valve in-line of your rear brakes to keep brake pressure at 10 psi in the rear.

If you want, I do have my 1 year old OEM ACDelco disc/drum MC I removed when I swapped over to rear discs (probably about 7k miles on it) just sitting in my garage. Oddly enough I planned on throwing it out months ago but never had the heart to do it since it was so new. If you're interested, I'll give it to you for $20 + shipping cost.

If you want to keep the nicer looking MC reservoir and not throw away the money on that shiny new MC, then you might want to consider installing one of these to fix your issue: https://www.summitracing.com/search/part-type/residual-pressure-valves?N=brake-line-pressure-retained-psi:10-psi

In addition, the cup seals in the wheel cylinders require the residual pressure to allow them to seal correctly.

 

[Gibson]

Whether you have a residual of 2,5,or 10lbs, or even zero lbs., it will make no difference in the pedal travel to engage the rear shoes.
The retracting springs always pull the shoes back all the way.
Assuming that their is ZERO air in the lines, the pedal travel will depend upon the diameter of the MC piston,, and how far the shoes are from the drum.
Make sure the rear adjustment is good.

 

[Schurkey]

GM eliminated the residual check valves from the drum-brake master cylinder in the early '70s.

If they ever returned to having a residual check valve, I don't know about it.

The O-N-L-Y purpose of a residual check valve is to maintain slight pressure on the wheel cylinder seals, so that they don't leak air into the system. They're better at sealing fluid IN than sealing air OUT.

Four common reasons for poor rear brake performance:
1. AIR IN THE SYSTEM!!!!!!!! Air in the master cylinder, air in the ABS, air at the wheel cylinders, air ANYWHERE in the hydraulic system.

2. The rear brakes are the horrible leading-trailing shoe design, and they're out of adjustment

3. Rear brake hose (or metal tubing, or seized/blocked pressure valving--proportioning valve, holdoff valve, etc.) blocks fluid flow. I've had multiple brake hoses swell shut--tighter than a bull's ass in fly season. No fluid gets through. If you bleed the brakes and get reasonable fluid quantity at the bleeder screw, this isn't the problem.

4. Someone tightened the park brake cable, when they should have adjusted the service brakes.

There are other causes, these are merely four common ones.




For the record, if you've got "Low Drag" front calipers, installing a non-Quick Take-Up master cylinder will destroy brake performance, and create a low pedal in the process.

Pretty-much all 1/2-tons, and maybe the six-lug 3/4 tons have low-drag calipers. The original master cylinder will have a large-diameter piston (36 or 40mm) at the rear, next to the booster; and a "step" in the casting to accommodate it.

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[Gibson]

GM eliminated the residual check valves from the drum-brake master cylinder in the early '70s.

If they ever returned to having a residual check valve, I don't know about it.

Perhaps on just the 1/2 tons?
When I converted my '80 K2500, (disc fronts, drum rears, with the 14 bolt FF,) to 4x discs there was a residual valve in the MC for the rear brakes.
I reached down into the MC with needle-nose pliers and pulled it out.
Edit; I said K2500, but in those years it was called a K20 for Chevy, or a K25 for GMC.

 

[Schurkey]

Was that a "Genuine GM" master cylinder, or an aftermarket "rebuilt"?

The service info I have says that GM quit using residual check-valves in the early '70s; but given the age of that manual, they may have gone back to residual check valves later.
Image 1: http://hbassociates.us/Delco_Brake_Manual_01.jpg

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Image 2: http://hbassociates.us/Delco_Brake_Manual_03.jpg

You must be registered for see images attach

 

[Ryan cin]

The point I was trying to make above, it that your master cylinder choice is likely not the reason your rear drums aren't working. Go adjust your brakes and bleed them before you shoot the parts cannon at it again.

Drums are adjusted, adjusted them out to where the drum barely slid on to test and they still sucked. Tried parking brake and locking the (only front wheels I guess) up going about 35 in reverse. Not sure what’s up

 

[Ryan cin]

GM eliminated the residual check valves from the drum-brake master cylinder in the early '70s.

If they ever returned to having a residual check valve, I don't know about it.

The O-N-L-Y purpose of a residual check valve is to maintain slight pressure on the wheel cylinder seals, so that they don't leak air into the system. They're better at sealing fluid IN than sealing air OUT.

Four common reasons for poor rear brake performance:
1. AIR IN THE SYSTEM!!!!!!!! Air in the master cylinder, air in the ABS, air at the wheel cylinders, air ANYWHERE in the hydraulic system.

2. The rear brakes are the horrible leading-trailing shoe design, and they're out of adjustment

3. Rear brake hose (or metal tubing, or seized/blocked pressure valving--proportioning valve, holdoff valve, etc.) blocks fluid flow. I've had multiple brake hoses swell shut--tighter than a bull's ass in fly season. No fluid gets through. If you bleed the brakes and get reasonable fluid quantity at the bleeder screw, this isn't the problem.

4. Someone tightened the park brake cable, when they should have adjusted the service brakes.

There are other causes, these are merely four common ones.




For the record, if you've got "Low Drag" front calipers, installing a non-Quick Take-Up master cylinder will destroy brake performance, and create a low pedal in the process.

Pretty-much all 1/2-tons, and maybe the six-lug 3/4 tons have low-drag calipers. The original master cylinder will have a large-diameter piston (36 or 40mm) at the rear, next to the booster; and a "step" in the casting to accommodate it.

You must be registered for see images attach

I bought new reman autozone calipers, I think C555 and C554. These are low drag, right? And my old master did have that step.


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[Ryan cin]

Thread confirms what I said earlier. Seems like everyone has a different experience and different views on how these brakes work and what causes problems. There’s so much variation in these setups it seems that all this conflicting info is right for certain trucks but wrong for others. Weird.

 

[SUBURBAN5]

Thread confirms what I said earlier. Seems like everyone has a different experience and different views on how these brakes work and what causes problems. There’s so much variation in these setups it seems that all this conflicting info is right for certain trucks but wrong for others. Weird.

That's why I did new oem. Look at rpo and get new hardware for that. However you need to make sure you don't have other brake issues. Are all you brake hardware fairly new or have been replaced?