Master cylinder under floor
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Laughing Gas
Joined: Sep 2008
Posts: 966
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From: Missouri
Master cylinder under floor
My stock single mc on my 53 has gone bad. I am going to replace it with a dual. I am staying with standard brakes due to lack of funds. Do I need to use residue valves? If so, what is the difference between the two?
Cargo Master
Joined: Jun 2001
Posts: 2,571
Likes: 22
From: Oxford, Indiana
Most master cylinders designed for use on a drum brake system will have a built-in residual pressure valve in the circuit to keep just a little pressure (~10 lbs. or so) on the wheel cylinders to keep the cups sealed to the cylinder bores and prevent air from entering past them. This is true regardless of whether or not it was mounted above or below the wheels in its original application. That much residual pressure will cause disc brakes to drag the rotors so there are no residual pressure valves in an OEM-type disc brake circuit with a firewall mounted master cylinder.
There are two circumstances when external residual pressure valves need to be installed. The first is when using a disc brake master cylinder with no integral RPV on a drum brake system. You need to install a 10 lb. external RPV in that case to keep the wheel cylinders from sucking air. The other circumstance is when using a disc brake master cylinder with disc brakes in an under-the-floor application where the master cylinder is at the same height or lower than the calipers. That situation can cause the brake fluid to siphon back out of the calipers to the master cylinder. You need to install a 2 lb. external RPV in that case to keep the fluid from siphoning. It isn't a problem when the master cylinder is higher such as it is on about any vehicle with OEM disc brakes.
With all that crap said, the correct answer for you depends on what kind of brakes you are going to run on each axle and what kind of master cylinder you choose. From the tone of your post, I assume that you still have stock drum brakes on both axles. If that is the case, I'd use a master cylinder from a drum/drum '67 Mustang. It's a reliable, inexpensive cylinder that has been proven to work well on these trucks. The bonus is that since it is set up for drum/drum, it will have internal RPV's already installed in both circuits and won't require any externally.
There are two circumstances when external residual pressure valves need to be installed. The first is when using a disc brake master cylinder with no integral RPV on a drum brake system. You need to install a 10 lb. external RPV in that case to keep the wheel cylinders from sucking air. The other circumstance is when using a disc brake master cylinder with disc brakes in an under-the-floor application where the master cylinder is at the same height or lower than the calipers. That situation can cause the brake fluid to siphon back out of the calipers to the master cylinder. You need to install a 2 lb. external RPV in that case to keep the fluid from siphoning. It isn't a problem when the master cylinder is higher such as it is on about any vehicle with OEM disc brakes.
With all that crap said, the correct answer for you depends on what kind of brakes you are going to run on each axle and what kind of master cylinder you choose. From the tone of your post, I assume that you still have stock drum brakes on both axles. If that is the case, I'd use a master cylinder from a drum/drum '67 Mustang. It's a reliable, inexpensive cylinder that has been proven to work well on these trucks. The bonus is that since it is set up for drum/drum, it will have internal RPV's already installed in both circuits and won't require any externally.
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