Rear brakes locking up before front - tips?

[Ekliptix]

I had my truck in the shop a while back. They found the diaphragm had a leak, so it was replaced. They added a preset proportioning valve.

Since driving the truck, the rear brakes lock up well before the front. I've been told adjusting the rear drums within the drum won't help.

My next step was going to buy a simple valve like this, to turn down the pressure doing to the rear brakes.



Any other ideas??

[kwmech]

Are the front brakes even working. Can you take a pic of the master cylinder plumbing that is not 5 times the size of life size

[Getter-Done]

Resize

[Steeveedee]

I got tired of monkeying around with the kit I bought to convert my truck to front disc brakes and removed all the prop valve stuff. I put in that Wilwood valve, and adjusted it until the brakes "equalized". In my case, that valve is wide open, because I have a lot of weight on the rear tires, which only increases when I tow my 6k+ pound travel trailer. The jury is out on brake wear characteristics, since I have driven it maybe 7k miles since I did the conversion.

[HO455]

Do you have front drums or disks?

Long bed, short bed, Suburban, Longhorn?

Under what circumstances do the rears lock up first?

I would definitely bleed the fronts and do a thorough inspection of them before throwing parts at the truck.

[Sheepdip]

Like HO455 says bleed the fronts including the M/C to make sure all the air is out.

[Warrens69GMC]

My 1978 did that, it was a leaking axle shaft seal.

[Ekliptix]

Good tips. I'll get the front in the air to see if the front brakes are even working. Then, bleed them.

It's a disk (front) drum (rear) setup.

On hard braking, the rears have locked up. It seems like the fronts are doing nothing.

[kwmech]

Possible that the valve is off center. Bleeding the front may bring it back unless it is stuck or there is another issue causing this

[RichardJ]

Pull the rubber boot off the Combination Valve. Have someone press on the brake pedal and watch the pin on the Metering valve to see if it moves out. If the front brake circuit is bled properly and the master is working properly, the pin will move outward.

The pin should be held in the OUT position when the brakes are being bled.

They make a tool for that purpose.
When the brakes are working properly, it takes only 50psi to push the Metering Valve pin OUT. When there is air in the lines. the air will compress before the fluid will reach the threshold pressure (~50psi) and open the valve so that the brakes can be bled.

DON'T bleed the MC at this point. All that will do is provide the possibility of more air entering the lines.

[Jason Banks]

I have the same problem on one of my trucks. 1968 and the front is converted from disc to drum... probably the same proportioning valve you picture.

I bought the Wilwood valve you picture, but have not installed it yet. I'm interested to see what you figure out.

On my latest truck, I went with the Willwood proportioning valve that has the built in adjustment, also a 1968 converted to disc. It was super easy to get a good adjustment that didn't feel like the back end was going to come around...

[Jason Banks]

I'm going to throw this out there also...
https://www.youtube.com/watch?v=g63mP4HwVEQ

Here's the CPP tool that I have.
https://www.summitracing.com/parts/clp-pv-tool

[68 P.O.S.]

If your brakes check out, the way I see it you have a couple options.

1. Keep it as is and deal with it.

2. Get an original truck combo valve since its already setup for the truck.

3. Take your existing combo valve, remove/disable the proportioning guts, and install the Wilwood prop valve inline in its place. This keeps the metering valve and brake warning circuit intact.

4. Remove the existing combo valve altogether and only use the Wilwood prop valve. This gets rid of the metering valve and brake warning circuit tho.