RPV clearification needed

Ok I have searched and read all I can find on this subject, but I still have one question. My brake system (9"x 1.75" f/r drums) came with no RPV's, I have changed to 12" Buick drums in the front and 9.5" GM drums in the rear. I modified the backing plates to use the original style wheel clyinders.

Is there any reason why the system would "need" the RPV's? I know that some say they are a must and some say they are a waist of time. I also know that they basically keep pressure on the shoes to maintain a better "feel" as the shoes ware, but are they needed and if so, 10# front and rear, or would 2# do it since the system was designed for and still is a four drum system?

 

The reason for a residual pressure valve is to keep fluid from draining back to the master cylinder. If the master cylinder is below the floor- below the wheel cylinders or calipers this will prevent you from having to pump the brakes to get a hard pedal when you first hit the brakes. If the cylinder is on the firewall these will not bre needed. Use a 10 pound for drums and a 2 pound for disc brakes.

 

Thanks. That is what I thought, but since the master was below the wheel cylinders from the factory, I was unsure. It is still below them and actually about 3" lower now. I'll pick up a set and be on my way.

Thanks again for the fast responce.

 

The RPV on a drum brake system is also there to stop air being drawn in past the wheel cylinder seals when you let off the brakes.

A factory drum master cylinder will have one built in to the cylinder whether it was originally mounted on the firewall or under the floor.

If you are using a master cylinder that came off a car with 4 wheel drums, you shouldn't need to add external RPV's.

 

get the cheap br*** ones, the expensive wilwood ones leak.

 

Most drum brake master cylinders have the RPV built in.

 

I had seen that there might be one built in, but never did find conformation on that. Thanks for clearing it up. I'll pull the master cylinder brake down and check the parts list to make sure before I order some.

 

Pull the line off the master cylinder, and try to insert a toothpick into the fitting, gently. If there is something in the way, it has a RPV.