Model A with master cyl under the floor. Bought a new dual resevoir master cylinder to fit an old Granada with a 7/8 bore. Manual brakes. Disc on the front and rear. Question...Do I need to use residual valves? Other people have told me to stay away from them, especially with disc front and rear. What's the low down?
this is from wilwood's info on speedwaymotors.com The two pound valve is used in disc brake applications where the master cylinder is mounted below the horizontal plane of the calipers and fluid drain back occurs from gravity and vibration, thereby causing excessive caliper piston retraction and a longer brake pedal stroke. The minimal two pound residual pressure prevents fluid from flowing back without causing the brakes to drag.
see how the brakes work first. if they don't feel right then install the valves. as a rule if the mc is below brakes you will need them. sometimes you get lucky though.
with the master cylinder below the floor you need to use them, they won't hurt anything and will keep the brake fluid where it needs to be
There is absolutely no reason "to stay away from" residual pressure valves in any brake system, even if the mounting of the M/C doesn't require them. They do no harm. The rear system should have a 10# valve by the way. What is worth avoiding, if you don't need one in a particular application, is the proportioning valve that many builders say you "must have" whenever you have discs front/drum rear. That is absolutely not always the case. Sometimes it is needed, sometimes, not. It depends on the particular combination of a variety of factors, some of which are, rotor diameter, caliper/pad size, friction coefficient of the pads, rear drum size, shoe width, wheel cylinder bore, tires size, both in diameter and width, tread compound, vehicle weight distribution........etc. The only way to know if your particular vehicle and it's combination of disparate parts need a proportioning valve is to assemble it without one and try it. Unless you are a senior engineer. So called "panic stops" should result in front wheel lock up before rears, if rears do at all. If rears lock first, directional stability is lost and a sideways skid will be the likely result. Ray
So I have a question on this subject.I keep hearing "Fluid drain back". Correct me if I'm wrong but would'ny there need to be a way for air to get into the calipers for fluid to flow out of them? I've done a few like we are talking about, My 51 Ford being one of them. It has Nova Disc front, drum rear, Nova drum/disc master and booster under the floor to stock pedal. No residual valves, been driving it 4 years. Hard braking maybe 1/3rd pedal travel and solid brakes. I don't half to pump them to get them up and there is No drag after stopping. This is not the only car I've done this to, why do I get away with it and others seem to have problems? The Wizzard
I've done my share and we used several different kits. Seems like every car ends up different in the braking dept. We all this makes no sense. Like Pist-N says his works great. I know of one car that has been put together with everything you can think of wrong and it will stop on a dime and does not have a seep are leak. I know of another that has the best money could buy and is still not working proper ?????????
.Quote: honestray You must have missed the part about front and REAR being disc, the 10# valve is for drum brakes. Yeah, I did............hate it when that happens
Do the pistons in the calipers actually retract? I thought they just released clamp pressuer when you let the pedal up. What would make them retract? The Wizzard
The theory is gravity will cause leakback to the m/c and that will cause the piston to retract. Don't know if would actually ahve enough force to pull the piston back, but I just finished my brakes with about the same setup and used residual valve for front and back just incase
A lot of misconceptions here: 1. Master cylinders for drum/disc set-ups are equipped with one residual pressure valve for the rear drum wheel cylinders. Even slight drum eccentricity wiggles the Bendix brake assembly enough to pulsate the piston seals to trickle in small amounts of air (even into those that have the cup expanders). Had a car that got the spongy pedal at about every 2000 miles - had to bleed the air out of the rear system - eventually discovered the factory had failed to install the residual pressure valve in the master cylinder. I installed one: no more spongy pedals. 2. Drum/drum set-ups use master cylinders that have two residual pressure valves installed. They are located behind the brass flare seat. This being the case, you do not need to buy expensive in-line residual pressure valves. 3. Disc brake piston retraction is miniscule: this is performed by the square (cross-section view) "O" ring. 4. Very little brake fluid actually flows through the lines during apply and release, providing your brake shoes are correctly adjusted. 5. Brakes never completely release. Pads maintain a very slight contact with the rotor and shoes maintain a very slight contact with the drum, but not enough to cause heat damage. This slight contact keeps the discs and drums clean with a continuing wiping action, necessary for best friction on the next application. 6. Ray is correct on proportioning valves. Read thread #5 again and consider all the variables that come into play. Again, avoid rear wheel lock-up.
Wish I typed with more than 2 fingers, I'd like to expand on the air in line comments. Personally I don't buy it or, I have not experianced it yet. Bled and air tight has always stayed bled and air tight for me. Maybe I'm just lucky as in I don't try to over think things. I've always known drum masters have a check valve in them. Why do you need to add another? A full master after bleeding with a proper fitting lid won't let fluid out so how can the calipers displace fluid out of them? As the pads ware, top off the master and the pedal movement will stay where it belongs. No leaks, no problems. This is "MY" way of looking at the brake system and until it stops working for me I'm sticking with it. To me in line residual valves are just gadgets and often over used. The Wizzard
The comments about needing a residual valve when the MC is below the plane of the brakes are right on the money. I experienced the problems described in those posts when I built my '39 Chevy. It drove me CRAZY until I figured it out. Add the residual valve = no more problems.
While factory M/C's are/were equipped with residual valves for the drum side, or both in drum/drum systems, it is my understanding that the current crop of aftermarket M/C's DO NOT have integral residual valves and they must be added if a builder desires to use them. Exchange rebuilt M/C's likely have whatever was correct for their original application and would not need auxiliary residual valves. It is also true that the disc brake caliper piston seals deflect slightly on brake application and return the piston to a slightly 'retracted' position and normally that is very close, or slightly dragging on the rotor. However, just as drum eccentricity can affect wheel cylinders, warped rotors, which are not at all uncommon, can change the 'at rest' position of the caliper pistons. For those that choose to not accept common engineering principles for whatever reason, rock on brother........but just 'cause you haven't experienced the potential problems it doesn't mean they don't exist. I've never been struck by lightning either, but it doesn't mean it can't or won't happen. Yeah, I still go outside in thunder storms in spite of knowing that fact. Ray
Other things to consider here. Usually the applications that have no residual valves and are not experiencing syphoning problems are due to the fact that the master cylinder bore size is adequate to replace the syphoned fluid in just one stroke of the pedal, an indication that the bore size is too big for the calipers in use. In addition you need to consider the fact that some day you may park your ride on a hill, nose up, which aggrevates the problem, same as when you jack it up to change the oil, etc. I always tell our customers if they are not sure that they need the 2 psi rpv, put it in anyway, they are cheap insurance. Remember, having to stroke the pedal a second time is not a problem, as long as you have time to do it. Other than the license plate, there is nothing on the front of your ride you can replace for the cost of an rpv.
As I posted originally I have disc front and back and the car came already plumbed with the valves so I have left them in there. I have bled the brakes the old fashioned way (pump and bled) and have all the air out that would come out. I do have brakes and can drive it and stop but the pedal goes way down before I get a good solid pedal. If i need to do a serious stop it will do it, I have tried. But I am still uncomfortable with the pedal going almost to the floor.
If the pedal travels really far, but does give a firm feel, and adequate breaking, then I believe that your master cylinder is too small. What are the calipers you are running?
My understanding is that master cylinders have not had residual valves in them since the mid 1960's. Most vehicles have an external combination valve that contains the residual valves and proporting valve. Take a small nail or paper clip and push in into the hole in the master cylinder where the brake line connects. You can feel the spring pressure if there is a residual valve. I had simular questions. I got this information from another HAMBer. My master cylinder is a drum/disk from and 81 and later ford. It has no residual valves.
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