Signs of a failing proportioning valve

[13matsc]

Hi guys,

Could these symptoms be caused by a failing or already defect proportioning valve?

- Brakes sticking intermittently when driving slow and more or less continuessly braking. When pushing the brakes hard it stops and releases, but comes back. Goes away after some driving.
- Seems like the pressure in the lines are building up and then causes super hard braking. The truck «slams» the brakes with normal pressure on the pedal.
- Sticks on the rear brakes only.
- Worse when the truck has been sitting for several days/weeks.
- All the brake components, except from front calipers and proportioning valve has been replaced over the last couple of years. Master cylinder twice as I thought this was the problem. Load sensing valve removed as I also first believed this was the problem.

 

[75gmck25]

I have a 3/4 ton K25 with 14 bolt FF rear axle, so pulling drums to inspect rear brakes is not easy. If I leave the parking brake set for anything more than a few hours, the rear shoes seem to take a set against the drums, and there is a significant clunk as they break loose. I usually have to back up first to get them to turn freely.

I think my rear shoes absorb moisture or are dirty, and that is why they stick. They were worked on and everything replaced several years ago, but probably don’t have more than about 15-20k miles on them since then. I have learned to use wheel chocks instead of the parking brake if I leave the truck parked more than a short time. YMMV

 

[13matsc]

Thanks for the input. I rarely use the parking brake as I park inside a garage on level surface, but I will take notice and monitor this.

 

[Old Guy Bill]

Sounds like the usual flexible brake line failure.. even if it has been replaced.
Todays parts aren't like the originals.

 

[13matsc]

Sounds like the usual flexible brake line failure.. even if it has been replaced.
Todays parts aren't like the originals.

You mean at the rear axle? Is barely a year old. I only drive the truck during the summertime and only 2k miles normally. Would be strange to fail so soon.

 

[idahovette]

Could have been crap when it was installed.........see Old Bill's post

 

[13matsc]

I installed this after the problem first occurred when I was removing the load sensing valve and needed to run a new line to the rear axle. The rubber hose and the hard lines were replaced at the same time.

 

[Camar068]

Sounds like the usual flexible brake line failure.. even if it has been replaced.
Todays parts aren't like the originals.

getting ready to do my brakes due to drivers side locking up when wet or really cold. I've been thinking if the ptfe hoses would eliminate this issue and still be flexible?

A thought to add, sorry for a 'lil hijack OP.

 

[Ricko1966]

The combination valve could have trash in it,it's not unheard of. It sounds more to me like you didn't check the clearance between the brake master and the brake booster.Was a master change just previous to when this started happening? If you hadn't said you just replaced all the brakes in the rear an even bigger suspicion would be gear oil on the brake shoes.

 

[13matsc]

The combination valve could have trash in it,it's not unheard of. It sounds more to me like you didn't check the clearance between the brake master and the brake booster.Was a master change just previous to when this started happening? If you hadn't said you just replaced all the brakes in the rear an even bigger suspicion would be gear oil on the brake shoes.

I replaced the master in late 2023 along with new wheel cylinders, shoes and hardware on the rear brakes. This started happening in the summer of 2025. Replaced the master again this week, and the problem still persist.

Can you elaborate on the gear oil?

 

[Ricko1966]

I replaced the master in late 2023 along with new wheel cylinders, shoes and hardware on the rear brakes. This started happening in the summer of 2025. Replaced the master again this week, and the problem still persist.

Can you elaborate on the gear oil?

If the rear shoes get gear oil on them they get sticky, they want to lock up,they want to stick etc. Master cylinders are suppose to have .020 clearance between the pushrod and the master cylinder,incorrect clearance will cause a low pedal or brakes that drag sometimes.