Back brakes lock before fronts

[meyer]

1977 C20 350
I recently bought this truck and its back brakes lock before the fronts. Not only this, but they lock pretty easy.

I have replaced the following with still no luck.
....................................
rear drum brake cylinders
rear drum brake spring
rear drum brake pads
rear drum brake drum
rear flexible hosing
front calipers
front brake pads
front disks
front flexible brake lines
proportioning valve
master brake cylinder.
...................................

With my luck of course its going to probably be an easy $5 fix

I need help as this is unsafe to drive.

Ideas? Thank you

 

[MikeB]

Did you get a good stream of fluid from the front calipers when you bled them? Just thinking it could be a dented or heavily corroded hard line between master cylinder and proportioning valve. Also, the truck had the problem BEFORE you did all this work, right?

For some reason, it sounds like you are getting too much pressure to the rears, or the fronts are taking too long to engage. Typically the proportioning valve reduces pressure to the rear, and a 2 psi residual valve holds the front pads close to the rotors (when brakes are not applied). I'm not sure if your truck had a front residual valve built into the proportioning (or combination) valve, but some do and some don't.

Are you sure the new proportioning valve is specific to your truck? Is it by chance a "height sensing version" installed at the rear of the truck? (I think these were used only on C30 trucks, but I'm not 100% sure.

For what it's worth, I have also seen one rear wheel (drum brake) lock up prematurely due to some type of contaminant on the shoe lining. Finally, are the rear tires in good shape? Not old and rock hard?

 

[Craig 85]

Once you determine there is no issue with the brake system, you can consider adding an adjustable aftermarket proportioning valve. My K30 has the factory load valve, I had an 87 K20 and 87 C30 that also had them. I'm not sure these were installed before 1981. I had a C20 3+3 that didn't have it, but I never had a lock up issue.

 

[Georgeb]

Make sure the lines are connected to the correct ports of the master cylinder. The larger resivoir is the front.

 

[HotRodPC]

Once you determine there is no issue with the brake system, you can consider adding an adjustable aftermarket proportioning valve. My K30 has the factory load valve, I had an 87 K20 and 87 C30 that also had them. I'm not sure these were installed before 1981. I had a C20 3+3 that didn't have it, but I never had a lock up issue.

This factory rear load valve was short lived. It was from about 84 to about 87ish in 3/4 and 1 tons only. I actually thought it was to 86, but you say your 87's had it too. GM recommends by passing that and not to use it.

 

[Craig 85]

I just removed mine last week while doing the rear brake rebuild. I did the same thing on both 87's back in the 90's. I hated the load valve.

 

[meyer]

All the tires are old and rock hard. Would old tires have that big of an effect?

 

[meyer]

Yes it did do this before all of these repairs, which is why we did them. The proportioning valve lined up perfectly and several major auto sites recommended the said type for my trucks brake systems.

Is it possible their is an unreachable air pocket that is 'stuck' somewhere?

 

[CSFJ]

All the tires are old and rock hard. Would old tires have that big of an effect?

Tires will make a huge difference.

 

[LGDH]

I have an older one a '77 and have encountered a few problems over the years. One was the drums being out of round, and they would grab, even after a short period of time after having them machined and even after buying new ones.
The other problem that can occur is there is a special tool that is screwed into the factory proportioning valve where the brake fail switch is, if this isn't used bleeding can be difficult, after bleeding is completed the switch is replaced again.
I got that annoyed I put discs on the back and have never looked back. This was on a chev 14 bolt diff.
This my first post, yeeha, so hope it is of help

 

[LGDH]

Just re-read the original post, mine is the same year as meyer's

 

[shiftpro]

All the tires are old and rock hard. Would old tires have that big of an effect?

They are super slippery... good for smoke shows and that's about it.

 

[highdesertrange]

did you install the rear shoes in the proper orientation? short lining to the front, long lining to the rear. highdesertranger

 

[82flatdeck]

This factory rear load valve was short lived. It was from about 84 to about 87ish in 3/4 and 1 tons only. I actually thought it was to 86, but you say your 87's had it too. GM recommends by passing that and not to use it.

Just as a note these go back to at least 82. My 82 has one that I need to be rid of at some point.

 

[theblindchicken]

Just for added clarification, at what speed will it lock up on you? How hard are you actually on the brakes?

Even with my 35's, I can lock the rears up on my truck pretty easily if I have to make a pretty solid stop. I've got 12.5" wide wheels, and I'll definitely bet its easier with skinnier tires too (definitely was easier with my 32x10.5's).

Drums are much more prone to locking up versus disc brakes. Also doesn't help any whenever the bed is empty too.

No weight in the rear, probably skinnier tires, heavy truck... We've got a few odds stacked up against us.