Piston Slap: Panel truckin’ out of proportion?

Gerry writes:

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?

Sajeev answers:

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:

1. Bleed the prop valve again, there’s a handy tool for GM style valves.
2. 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?