Why the **** won’t my rear brakes work??
 

Need some serious help here, about to drag the truck out in my driveway and burn it over as much trouble as these brakes have been giving me. I’ll give a recap of what’s been done on this truck.

1978 Chevy K20. D44HD front end. Corporate 14 bolt rear end.

Originally a vacuum boost truck. Originally had 11” rear brakes.

Swapped from vacuum to rebuilt hydroboost unit from a 1983 CUCV.

Installed a new Master Cylinder for a 1993 Suburban K2500 (aluminum body/hydroboost)

Installed new spindles, rotors, pads, calipers, hoses, and lines on the front.

Sourced 13” backing plates and changed over to 13” brakes. Installed new wheel bearings, drums, shoes, wheel cylinders, hardware kit/springs, parking brake kit, and all new parking brake lines in the rear. Also replaced the rear brake lines and brake hose.

Went to adjust the rear shoes. I can tell the new drums are slightly out of round, as they drag slightly on the shoes on only about 1/2 of the drum. Both sides do this. However, at the adjustment sight windows at the bottom of the backing plates, the shoes have a small gap and don’t touch the drum. I could likely crank on the adjuster and force them to, but the drum would be locked up. It feels like the shoes touch the drum at the top before the bottom. The driver side is slightly better than the passenger side. Currently, I have it adjusted where there is some slight drag.

After attempting to bleed the brakes, the front worked fine. I could not get consistent pressure to the rear, although the rear brakes did work really well once sporadically. I determined the proportioning valve was shot. Ordered a new proportioning valve and the bleed lock tool for it. Installed both of those and had better results, but still inadequate pressure in the rear.

Warrantied out the brand new master cylinder for another one and rebled the system. Now I have good pressure to the rear. However, the hose from the frame to the axle was machined incorrectly, and the hard line wouldn’t seat and kept leaking. This damaged the new passenger side hard line. I warranted out the hose and reflared the hard line and replaced the fitting. Went to install and the drivers side threads pulled right out without even tightening down, damaging the driver side hard line. Replaced the hose again, with a different brand, and reflared the other hard line and replaced that fitting. Installed the new line and rebled the system again. Removed all the air and the pedal feels good.

However, if I have the pedal pressed I can still turn the rear drums by hand. They definitely drag more than what my adjustment was, but they definitely don’t work. Front seems to lock up solid and work great. For an extra brain scratcher, if I manually pull the parking brake cables by hand, both rear brakes will work perfectly via the parking brake.

I’m completely at a loss here.

Re: Why the **** won’t my rear brakes work??
 

They should self adjust. Go in reverse about 10mph. Stomp on the brakes. Repeat a few times. The hard stop in reverse is the key.

Re: Why the **** won’t my rear brakes work??
 

Self adjusters are working fine. I guess to clarify, it acts like the shoes are making contact first at the top of the drum (instead of at the bottom first like they should), coupled with the fact that the wheel cylinders don’t seem to be moving the shoes out much if at all, despite having good pressure in the system.

Re: Why the **** won’t my rear brakes work??
 

Just for reference in case anyone can point anything out, these were taken just before the drums were installed.

Driver:

https://i.postimg.cc/wxtL5yWt/IMG-5972.jpg

Passenger:

https://i.postimg.cc/xCZjTdHh/IMG-5968.jpg

Re: Why the **** won’t my rear brakes work??
 

So I managed to find an obscure thread on another forum where several members had ran into this same problem after rebuilding their 14BFF 13” drum brakes. Apparently, it was noticed that the front lining is supposed to be the thicker lining, even though normally it is also the shorter lining. After swapping the thin/short shoe to the rear and putting the thick/long shoe to the front, they all reported the issue to be solved. Thoughts or opinions on this?

Re: Why the **** won’t my rear brakes work??
 

I found the following note in the Technical Manual for CUCVs. Covers both the M1009 Blazer and M1008 K30 versions.

NOTE
Primary brake shoe (17) has a
shorter lining than the secondary
brake shoe (5), and is always
installed toward front of truck.

Probable that the K20 is the same.

Re: Why the **** won’t my rear brakes work??
 

Does it have one of these stupid "load-sensing" things on it? GM advises removal and bypass...

https://rixsquarebody.blogspot.com/p...ommending.html

Re: Why the **** won’t my rear brakes work??
 

Nope, nothing like that. Regular proportioning valve setup.

Re: Why the **** won’t my rear brakes work??
 

Randomn thoughts to throw at it;
What size rear drums go with the vehicle you got the master from? Aka, is the master able to push enough fluid for the larger drums?
I never looked into it on GM since it was never an issue (just fords) but where is the residual valve in the original setup, and where is it now? (part of the master in the original, separate screw in valve in the donor system? something of that nature is what I'm getting at).

Re: Why the **** won’t my rear brakes work??
 

Same size. The master cylinder is a Napa M2904 that fits 1988-1994 Chevy/GMC trucks and Suburbans with hydroboost. This master cylinder also has the same specs as the OE 13” brake master cylinder, really the only difference is that this unit is aluminum instead of cast iron.

Re: Why the **** won’t my rear brakes work??
 

So have you verified the shoes are in the correct orientation?

Re: Why the **** won’t my rear brakes work??
 

Yes, I have pictures posted above for reference.

Re: Why the **** won’t my rear brakes work??
 

So this is what I was told by a long time Chevy tech:

This condition is somewhat normal to run into on these trucks. Don’t worry about the runout on the drums, just adjust the shoes like normal on the spot that they don’t drag (I.E., they should be difficult to turn by hand but should still turn with the wheel mounted) and take it for a test drive and bed the shoes in. Check the adjustment afterwards and he says it should be good to go.

It’s absolutely pouring rain this weekend, so I’ll tackle this when it’s nicer out.

Re: Why the **** won’t my rear brakes work??
 

Okay, I’m back at this again. Still can’t get the brakes to work. So far, I’ve gone as far as having adjusted the brakes so that the shoes drag heavily (wheel takes significant effort to turn by hand), but the pedal still won’t stop the wheel from turning even with the brakes fully applied. However, the parking brake functions beautifully. My observation is that the rear trailing shoe will adjust to the drum. The front shoe always retains a gap between the shoe and drum unless the parking brake is applied. My conclusion seems to be that the drum and front shoe have proper contact, but for some reason the wheel cylinder is only applying pressure to the rear shoe, or possibly not at all. I don’t have a way to check actual pressure seen at the wheel cylinder, other than the fact that the pressure seems to be adequate during bleeding. How can I have pressure at a wheel cylinder and still have no brakes?

Re: Why the **** won’t my rear brakes work??
 

I've had trapped air that was difficult to remove on my T400

Not sure what the difference was but I didn't have these issues with the cast iron 1979 K30 hydroboost master cylinder I used on my 1976 K20.

Re: Why the **** won’t my rear brakes work??
 

So I gave up trying to diagnose this externally. Removed the drum assembly and inspected the brakes again. Everything appears fine, all parts are correct and installed properly. Ran a ratchet strap loosely around the shoes and had a friend apply the brakes. The brakes have great pressure and a good amount of travel. The shoes themselves have witness marks where they’ve contacted the drum and there’s not much area where there is actual contact being made. However, I removed both shoes and fit them to the drum, thinking that maybe the arches weren't the same, but the shoes both fit the contour of the drum damn near perfectly.

As a final observation, we can get the rear wheels to stop turning by hand with the brakes completely applied, but it’s further along in the pedal travel where normal braking would occur - it’s more down in the “panic stop” area.

Master cylinder was a thought, but it’s been replaced twice and shares the same specs as the original. It even is meant for the exact same rear brake setup, just on a newer truck.

Re: Why the **** won’t my rear brakes work??
 

Maybe you need a smaller bore master.

Re: Why the **** won’t my rear brakes work??
 

Since you switched from 11" to 13" brakes, any chance the parts place gave you wheel cylinders for 11"? There might not be any difference but if there is and they did, could be a problem.

Re: Why the **** won’t my rear brakes work??
 

I think my only option would be to use an 11” drum Master then, as those spec out at 1.125”. My current master cylinder is 1.25”, which is also what the stock cast iron was.

The wheel cylinders are correct for 13” brakes, and have a 1.0625” bore. The 11” brakes had 1” bore wheel cylinders.

Re: Why the **** won’t my rear brakes work??
 

I have never had to do it but some people put a gauge on their brake line to test brake line pressure. It sounds like it isn't getting enough brake line pressure but I would have no idea why.

I never liked hydraboost in the past but I have it on my 86 motorhome and it seems to work quite well. I would think a K30 would use the same master cylinder as a P30. If not I would try something out of left field like try a master cylinder from an 86 P30. Good luck

Re: Why the **** won’t my rear brakes work??
 

Lots of 1 inch or smaller bore m/c available.
Look here.


https://leedbrakes.com/p-27639-about-us.html

Re: Why the **** won’t my rear brakes work??
 

Feel free to correct me if I’m wrong, but wouldn’t I be wanting to go larger bore, like 1-5/16” off the 1 ton trucks? Reason being if I’m into the pedal around “panic stop” area, and building pressure, that sort of indicates to me that I’m not having problem building pressure, I’m possibly just lacking volume. The larger volume will fill the wheel cylinders quicker (engage higher up in the pedal travel), but increase the pedal effort, which shouldn’t be an issue because I have the hydroboost. Am I thinking this over correctly?

Re: Why the **** won’t my rear brakes work??
 

Bigger bore equals more volume but less pressure.
Smaller bore equals less volume but more pressure.

That info comes from my motors manual.
Also read that on Hemings, mppower and others.

Re: Why the **** won’t my rear brakes work??
 

Based on this, it is possible your brakes are working closed to as designed. Before ABS, rear brake effectiveness on trucks was limited to prevent the rear brakes from locking up and the rear end trying to swap ends with the front during heavy braking. There was a period when trucks had rear only ABS to allow better braking without the cost of 4 wheel ABS.

Re: Why the **** won’t my rear brakes work??
 

There is contact points on the high sides of the shoes. Is the backing plate smooth/clean/shiny for the shoes to work back & forth?