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Hagerty Employee

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...


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Pit Crew

I have CPP disc brakes on my 1964 Dodge Polara 500. I put 11 inch drum brakes in the rear. I had weak fluid flow to front calipers during the bleeding process. My proportioning valve has a brake light switch mounted in the middle of it. I pulled the brake light switch out and replaced it with a brass plug. Full fluid flow at all four corners of the car. I took a closer look at that switch, and felt that the switch plunger was some what on the long side. I milled off approx 7-8 thou off of that plunger and reinstalled it. bled the brakes again and have full fluid flow to all four corners of the car. my thought is that this may not be the correct brake light switch for my application. if the plunger was to long it was most likely keeping the proportioning valve from centering itself and only allowing most of the brake fluid to travel to the rear brakes. I hope this helps you. 

Community Manager

Thank you for sharing, I never considered the brake light switch being the wrong one for an application, but that does happen when you have aftermarket upgrades designed for multiple applications!  Thank you again. 

Advanced Driver

Is the master cylinder plumbed to the valve correctly (correct reservoir feeding correct port on combo valve?) Is the combo valve plumbed to the chassis correctly (L/F - R/F - rear ports feeding correct positions?) Are the correct size hard lines being used (3/16" front, 1/4" rear?)

A large amount of fluid observed during bleeding would indicate that the master primary reservoir is feeding that position. As the system should be calibrated to provide a 70/30 front/rear braking effort, it appears that something isn't plumbed correctly or there might be an incompatible combination of parts in this application.