Piston Slap: Panel truckin’ out of proportion?
I have a 1953 Chevy panel truck with power disc brakes on all four corners, 9-inch dual diaphragm master cylinder, and a CPP proportioning valve. I am not happy with the braking power.
When I bleed the brakes I get lots of fluid from the rear brakes but very little from the front. I have checked the proportioning (combination) valve to make sure the piston is in the proper position and all seems OK. Any advice on my problem?
Because proportioning valves manage the flow of brake fluid between front and rear channels, that’s likely your issue. There could be an issue with the piston/bore in the master cylinder, but I’m thinking the prop valve is likely the problem. They are a bit tricky to work on, but digging into one isn’t like tearing apart a transmission, so let’s dive in.
Assuming there’s no obvious signs of leakage, either the valve has an air bubble or another physical blockage keeping the internal piston from cycling correctly. While it’s a huuuuge stretch, the latter could happen because of a jammed pin on the brake light warning switch (pictured here). Unlikely, as you verified the piston’s operation, but we are running out of options at this point.
I see two courses of action:
- Bleed the prop valve again, there’s a handy tool for GM style valves.
- Call CPP directly (tech hotline here), spare no details, and see what they suggest.
I am hoping the valve just needs to be bled. Braking performance is paramount in any vehicle, especially a classic pickup. What say you, Hagerty Community?
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