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

[Nick_R_23]

I can’t ever recall having a set of drums that didn’t feel like they worked well. The proportioning valve should also balance the brakes between front and rear.

The backing plates are in good condition and didn’t have any wear or grooves that I saw, but I will double check to verify.

Here’s an interesting bit of info I found out: On the 1 ton trucks, they use a 1-5/16” master cylinder with 13” brakes (difference being 3.5” wide shoes vs 2.5” wide on 3/4 ton) BUT the wheel cylinder used is the same. Honestly, I think I will try a 1-5/16” master cylinder, which should give me a higher pedal and move a higher volume of fluid through the system, which I’m thinking this truck needs. If I can get shoes to feel like they properly actuate against the drums, then I may still just have to let the shoes wear to the drum and adjust again after a few miles of use.

[hatzie]

That sounds like a plan.
I used the K30 hydroboost master right from the get go on my '76.

[Nick_R_23]

Makes sense to me also, I’ve heard quite a few people say that the K30 brakes work very, very well, while the K20 brakes just seem average. Dead Parrot may be right and this is just as good as the K20 brakes ever worked. If the larger master cylinder is what makes these shine, I’m all for it.

[Pontiac1976]

If I remember right the front of the drum is your parking brake side should be the thinner shoes. As the

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

Parking may have to be adjusted for that side.
are your wheel cylinder shaft the same length as the old one.

[Pontiac1976]

Something bother me about your right rear pivot spring on the e-brake hardware side.

[willem445]

You seem to be going through the exact same thing I am right now on my 1980 Chevy K20.

I've:
- Rebuilt rear drums brakes, both sides
- New hard lines on rear axle running to drums
- New master cylinder, properly bled
- New front calipers
- New front pads
- New front hoses

I've tried bleeding multiple times at each step, still crappy pedal after replacing all that. I am also suspecting something with my rear drums. The front are bleeding nice and seem to make good contact when pressing the brakes.

My rear drums are worn down pretty good and also might be out of center. I redid the rear drums many years ago so unfortunately don't remember what P/N shoes I bought. Drum cylinder seems to be fine as well testing without the drum on.

At this point, the only "quick" fix left is replacing the rear brake hose, maybe it's collapsed? Otherwise I think it's time for new drums and shoes again.

But after pricing out the cost for drums, etc., I might just do a Lugnut 4x4 disc conversion. Tired of these drums.

I feel your pain!

Here is a pic of my rear drum rebuild for reference

[v30crewcab]

Originally Posted by willem445

You seem to be going through the exact same thing I am right now on my 1980 Chevy K20.

I've:
- Rebuilt rear drums brakes, both sides
- New hard lines on rear axle running to drums
- New master cylinder, properly bled
- New front calipers
- New front pads
- New front hoses

I've tried bleeding multiple times at each step, still crappy pedal after replacing all that. I am also suspecting something with my rear drums. The front are bleeding nice and seem to make good contact when pressing the brakes.

My rear drums are worn down pretty good and also might be out of center. I redid the rear drums many years ago so unfortunately don't remember what P/N shoes I bought. Drum cylinder seems to be fine as well testing without the drum on.

At this point, the only "quick" fix left is replacing the rear brake hose, maybe it's collapsed? Otherwise I think it's time for new drums and shoes again.

But after pricing out the cost for drums, etc., I might just do a Lugnut 4x4 disc conversion. Tired of these drums.

I feel your pain!

Here is a pic of my rear drum rebuild for reference

you have both long shoes(lining) in that pic. you need a short shoe(lining) in the front of each for them to work correctly.

[willem445]

Originally Posted by v30crewcab

you have both long shoes(lining) in that pic. you need a short shoe(lining) in the front of each for them to work correctly.

Ahh that is good to know! After comparing pictures, I see exactly what you mean.

[cerial]

Did you ever replace the combination valve?

Sometimes you will get a a pocket or air in the valve. Pulling the brake switch and giving a light push can remove the air. You then install the new switch in wet and bleed the brakes.

[Nick_R_23]

Dang, it’s been a while since I’ve been able to get back to this. For those that asked, yes, literally every single piece of this system is new. Hoses, prop valve, whole 9 yards. So here’s what I’m going to attempt as a fix. Stock cast iron K30 1-5/6” bore master cylinder. Wish me luck, I’ll report back!

[Nick_R_23]

Huh, this is an interesting side note…K20/K2500 and K30 masters and hydroboosts don’t appear to be interchangeable. I’ve had to run a spacer with every K20 or K2500 master cylinder so far on this K30 hydroboost. Guess they are their own animals.

[hatzie]

There's a tool to lock the combination valve in the centered position.
You finesse the shuttle in the combination valve to the centered position with the warning lamp off then unscrew the warning lamp switch and install the tool.

https://www.amazon.com/Team-Performa.../dp/B07BYVK7SN

[LT7A]

Good luck, I hope you get this sorted. Then how about an update on your fleet? You had a cool assortment of projects going on.

[Nick_R_23]

Thanks, hoping to bleed the brakes next chance I get and see how it does. If this doesn’t work, my only thought is to replace the hydroboost unit with a new one as I’m on the fence as to whether this is really a good functioning unit or not. Doesn’t seem like it kicks the brake pedal back on startup much at all if it’s pressed and held, which is what many places state can be an indication of a faulty unit.

I’m sure the fleet has changed quite a bit. Currently it’s this ‘78 K20 which is just about roadworthy aside from the brakes and a couple really small things, the ‘74 Nova which I’ve now sourced a -638 roller block which is at the machine shop getting billet splayed mains and a Molnar 3.75” crank and rods and Mahle forged pistons along with the 4L80E on the stand ready for a ProBuilt kit with all the Sonnax goodies, and the Exocet is still kicking around too - some of the large parts like coil overs and rear subframe have been on back order for like 2 years now. We’re also potentially signing on a low mileage, one owner 2017 GMC Sierra 2500HD within the next week or so. I guess you can’t take the money with you LOL.

[GSP7]

Did you replace the rubber section part of the rear brake lines that attaches at the center of the rear axle at the differential? Those things can get plugged up



.

[c30]

I would have replaced the rubber hose to the axle in a heartbeat going from past experience.
I will never buy another chevy truck with the hydro boost system. Had a 2001 C30 and never did get the rear brakes working. Even took it to the local Chevy truck dealership and they could not get it to work.
This truck had the load sensing valve inline to the rear brakes. I pleaded with the dealership to just bypass the frickin thing, but they would not alter the factory system. And have you ever seen all the leakage from the hydro boost, under the hood?
Went through front brakes alot before I got rid of that POS.
Dump the hydro boost!

[hatzie]

Originally Posted by c30

I would have replaced the rubber hose to the axle in a heartbeat going from past experience.
I will never buy another chevy truck with the hydro boost system. Had a 2001 C30 and never did get the rear brakes working. Even took it to the local Chevy truck dealership and they could not get it to work.
This truck had the load sensing valve inline to the rear brakes. I pleaded with the dealership to just bypass the frickin thing, but they would not alter the factory system. And have you ever seen all the leakage from the hydro boost, under the hood?
Went through front brakes alot before I got rid of that POS.
Dump the hydro boost!

I have a 2005 K2500 T800 & 2000 C2500 T400 with stock hydroboost that works great.

My 1976 K20 had K30 hydroboost brakes and that worked great as well.

None of my trucks have leaks from the hydroboost.

The dealership techs that can't get the GMT800 brake system to work properly are darn poor excuses for techs. If the hydroboost is leaking then it needs to be rebuilt.

[Nick_R_23]

I have a new hydroboost unit on order. I’ve been through absolutely everything else multiple times and this is the only part that remains questionable. It doesn’t seem to react the way it should according to diagnostic procedures, so I’m replacing it. It may be a couple weeks before it shows up, but hopefully this can be a finale to this problem.

[mongocanfly]

i agree with Hatzie...you needed different people working on it.. hydroboost is the best braking system ive ever owned..
but if you dont like it ,thats ok to

[KQQL IT]

Not a hydroboost fan.
Rather a vacuum or manual.

Has anyone mentioned resetting the proportioning valve.
Chased no back brakes on my 73 for awhile. Took the prop valve apart, manual reset. Instant brakes

[AussieinNC]

Pull the switch out of the proportioning valve and stomp hard on the brakes...

This will put the valve back in its standard position.

Then replace the switch.

Short shoe front always...

Hope this helps

Bill