Rear Brakes only in Quick stop (proportioning valve?)

[fiddler]

i have a 1974 chevy C10 stepside.  today I had to do a quick stop and the rears locked up.  Seems I had no brakes on the front.  This seems to happen only in a quick or emergency stop mode, just when you need all the braking the most.  Is this possibly a proportioning valve issue.  Brakes were recently overhauled to include rotors, drums, calipers, brake cylinders, master cylinder, and vacuum assist for power brakes.  Everything was good 3000 miles ago.  I suspect the proportioning valve as it seems to be the only thing not new on the brakes.  Also...any tips to changing it out would be appreciated.  I suspect just disconnect the lines and wire and install the new valve and then bleed the system??  Thanks for any input..

[ChrisD]

Hi,
i had the same problem, the rear brakes had to much power after overhauling/renewing everything.
At one forum i got the answer, that it‘s the proportion valve, but that i had to „try and error“ to find the right one.

At the end, my mechanic did the same than in his own cars and put an adjustable proportion valve into the rear brake line.
At valve like that: https://www.jegs.com/i/JEGS/555/63022/10002/-1
In the middle of the truck is a junction where he put it between.

Regards
Chris

[TexasRed]

I had an issue with return springs on the rears. I think they were going bad and they had VERY few miles. Same as you, I replaced pretty much everything before this happened. The proportioning valves, you'll either have to get yours rebuilt or potentially go adjustable.

I would start with the cheapest/easiest thing first. The return springs are usually in the rear brake hardware kit. Those aren't too expensive.

[Mike81K10]

You can buy the valve at Classic Trucks if this forum store does not have them.

[VileZambonie]

You have a combination valve that incorporates a proportioning valve. The proportioning valve is designed to limit the pressure to the rear brakes so that they don't lock up as easily but it by no means will prevent them from locking up when braking overcomes the tire's traction. The metering valve delays the application of the front brakes until the pressure rise meets the value of the pressure needed to overcome the seat pressure of the return springs for balanced braking.

Start by inspecting everything, and if all is good, replace the rear brake hose and flush the system.

[fiddler]

Thanks for your help.  I have ordered a proportioning valve that says it is for my truck.  You guys having to trial and error gives me pause.  Hoping the one I ordered fits and cures.... 

[TexasRed]

The combination valve doesn't usually go bad and they ones that I've seen for sale I believe are the "universal" ones for front discs/rear drums but they're not really that great.