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ScottRods2945 01-03-2010 05:29 AM

54 chevy power brakes help
I am getting a 54 chevy 2dr sedan and right now it has original drum manual brakes all around. I was wanting to replace the master cylinder with a power brake booster and master cylinder kit. like a 7inch dual diaphram kit from speedway for example. but my question is these kits all say they are suited for disc/disc or disc/drum applications, I dont have the money to upgrade to disc brakes in the front, so will one of these kits still work well for my drum/drum application? If not then why? and what will work? I'd really like to have power brakes and disc brakes would be ideal but I don't have alot of money to spend.

redsdad 01-03-2010 06:33 AM

Will an 8 inch dual fit? A 7 inch dual on a car that size may be marginal.

Will this be an under floor mount? If so, you will need a 2 psi residual pressure valve for disc brakes when or if you eventually go to them. Buy the disc/disc unit and put two in-line 10 psi residual pressure valves in. Then when you do swap over to discs you can pull out the 10 and replace it with a 2. If it will be a firewall mount, you won't need the 2 psi units for the disc, but you can still use the 10 psi units for the drums. Replace the 10 psi units with unions when you go to discs.

redsdad 01-03-2010 06:41 AM

Sorry, didn't tell you why.

Drum brakes rely on a one way seal at the cups in the wheel cylinders. The springs pull the shoes back. There is a possibility for the seals to relax when there is no pressure applied to them allowing air to enter the wheel cylinder. The next time you apply the brakes, you have a spongy pedal due to the air in the system. A 10 psi residual pressure valve keeps just enough pressure in the wheel cylinder to keep the air out.

Disc brakes rely on a square cut o-ring to seal the caliper. The flex of the o-ring will pull the piston back a couple thousandths of an inch so the pads do not wear when the brakes are released. The square cut o-ring seals equally inside and out. A residual pressure valve would cause the pads to wear prematurely.

When you have an under the floor master cylinder on disc brakes, the calipers can be higher than the MC. With nothing to restrict the fluid, it can flow back into the MC, pulling the caliper piston back into its bore. The next time you hit the pedal, it goes to the floor. A 2 psi residual pressure valve is enough to prevent the back flow but is not enough to keep the pads applied.

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