Rear brake seem to be grabbing harder than fronts

[buddy350]

1986 1500 4x4 suburban

Added 8 lug 3/4 ton complete set axles to front and rear and now rear brakes grab and lock up if too hard on the brakes.

Not sure if this burban had this problem before because i never drove it with the 6 lug axles.

Today i replaced the front calipers and tried reseting the proportioning valve.

Bled all the brakes all the way around.

Used a snap on brake fluid vacuum on all of the wheels.

Cracked open the rear brake side of the proportioning valve and smashed the crap out of the brakes in hopes of centering the proportioning rod which i doubt was ever the problem.

Now im looking more at replacing the master cylinder and possibly backing off the rear shoe adjustment.
Just dont wanna back off too much and lose the e brake.

All and any suggestions are much appreciated.

 

[Tank6x2]

I don’t think you’ll lose the parking brake by adjusting it, but adjust both sides until they turn freely and ensure all the linkage points on the internals are all lubed and free.

 

[Turbo4whl]

Some 3/4 and 1 ton trucks had a second proportioning valve for the rear axle. It would limit the rear braking when the truck was empty. Controlled by a rod that let more pressure through when the springs were loaded and compressed. (ride height)

You can get a manual valve with a dial to limit the rear braking since the 3/4 ton axle has more braking than you need. Not automatic like the factory, but you can adjust it until you get the braking performance you need.

 

[Blue Ox]

What @Turbo4whl said. LSPV or Load Sensing Proportioning Valve. The trucks with 13" drums are quite aggressive when unloaded. Unfortunately those valves and their linkages are scarce today. So you'll probably have to use a manual one as suggested.

 

[Grit dog]

^What they said.
My truck still has a working LPSV. (Not hooked up). The difference is very noticeable when it is not regulating pressure.
Mine randomly goes into full load mode and then it stops very well!
With the big tires I actually prefer full pressure to the rear brakes but can see how they would lock way too quick with smaller or high pressure back tires.

 

[fast 99]

Type of friction material has an effect. Found that out through trial and error. Less expensive [softer] linings that wear faster will move more of the braking action to that axle. Brake balance front to rear can be adapted to a certain degree with linings. Same with front pads. I also changed the wheel cylinder bore size. Larger bore more braking.

 

[Blue Ox]

I tried reducing wheel cylinder bore diameter when I put the C6P brake system on my Suburban. Didn't make a damn bit of difference until I put a LSPV in the system.

Not saying that's the only answer by far. Other people's mileage has varied.

 

[Kilian]

Don't mean to hijack this thread but question is related. What helped proportion front to rear before the separate LPSV on the rear axle was used? Was a different front proportional valve used that did that?

 

[Blue Ox]

There's a combination valve on the front crossmember of all of these trucks. The LSPV is added to the ones with the big brakes.

 

[Kilian]

Yes, that I understand. Just wondering what changed in 82-83 that prompted the addition of the LSPV on the rear axle of the bigger trucks.

 

[Blue Ox]

Ah, I see.

I'd have to dig through the spec books to see where these things emerged, but I'd suspect that the C6P package emerged with the '81 redesign that brought about the 9.5" axle and the NP208 X-fer case.

I'm fairly certain that's when the frame modulus changed in the K20s from singular 20=3/4 ton to a 20 and a 20 HD. Prior I believe it was just 1/2 ton, 3/4 ton and 1 ton. a.k.a. 10, 20, 30.