The Stovebolt Site Forums The Shop Area Driveline weird. front discs bind with residual valve

hi guys

i installed a front disc conversion from classic

underfloor booster with combo valve and 2ib front 10ib rear residual valves


brakes are great but fronts bind/ rub a bit

i opened the bleed screw on fronts pressure is releaved and brakes unbind


i suspected it was the booster connecting rod adjustment but it made no difference

i checked if it was the booster to mc rod length but thst also msde no differeence

finally i removed the front resual valve and now theres no binding.....


has anyone experienced this before?


is it ok to use it without frint residual

This might help....

http://www.mbmbrakeboosters.com/ind...y&layout=blog&id=8&Itemid=16

thanks. im aware its recomended to use a front residual valve but when i do i get front brakes binding. so not sure whats going on....


not sure its safe or not to use ....they work , no rubbing though



im wondering now if the master already has a residual valve in its front linevoutlet. if it does that would explain it as id be doubling the resudual pressure in front line with the external valve....

I have been using disc brakes on my '37 for 16 years without residual valves. The master cylinder is in stock location also. Mine is manual disc up front, drum in the rear, stops just fine for me, no extra peddle travel or anything weird.

Joe

bit of an update;

got feedback from POL to check some things,

1. check booster to master cylinder rod gap =correct gap
2. check pedal to booster rod length (zero preload)

after this was done i still had residual pressure making the front discs rub.
after pressing then releasing the brakes the fronts are very hard to turn by hand at first and slowly get looser over perhaps one min. but even by then its not possible to `spin`the wheels by hand

(when i remove the front residual valve i dont get any rubbing)

im really at a loss to work out whats going on.

no matter what i do so long as i have the residual valve there i get brake rubbing.....

I even (without disconnecting fluid lines) unbolted the master and pulled it away from the booster so theres zero chance of any preloading and still the brakes stayed energised

i took the residual valve off and tested as best i could how much air pressure it would open against. it was only a rough test but by the air pressure gauage it appears to me that it took way more then 2ibs, more like 10lbs to get it to open. Im wondering if its a faulty residual valve? perhaps its sticking closed and giving too much back pressure?


maybe ill just fit a higher mounted remote reservoir and then do away with the front residual valve all together......

The high mount reservoir looks like the simplest option. You could also have a faulty or mis-labeled residual valve. Think about how the hydraulics work- - - -if the caliper piston has an area of 3 square inches, for instance, 2 pounds of line pressure will put 6 pounds of push against the brake pad. If the residual pressure goes up to 10 PSI, that push on the pad becomes 30 pounds of force. How about temporarily installing a 0-50 PSI pressure gauge in the front brake line and actually measuring the residual pressure? Just don't press the brake pedal hard, or run the engine to make the booster work- - - -you could damage a gauge that reads low enough to get a usable pressure reference.
Jerry

Residual valves are only needed when the master is below the caliper. A lot of kit makers use residual valves to compensate for a small diameter master to get higher line pressure, Using a small master gives a low pedal and a residual valves help eliminate that but often times does what your saying, dragging the caliper. remove the valve, the reservoir placement has nothing to do with using a valve, its the cylinder height that determines if a valve is needed.

hmm not sure about that.

wouldnt the static head (pressure) at the caliper at rest depend on the total head?. i would have thought thats going to depend on the total fluid height which is the top of the fluid level in the reservoir (if remote) or in the m/c reservoir if no remote to the calliper?

just to add some confusion, i got some further follow up from pol saying that with the proportioning valve I dont need a front residual valve......

need to look at that a bit...perhaps the metering part of the proportioning valve closes under its spring pressure before the back brakes do and therefore leaves a bit of a residual front pressure there and stops any bleed back to mc?

marked in red here -> http://drmike.smugmug.com/Other/Truck/i-XbN3T6s/A

certainly looks to my eyes like its closing off the front lines at rest.

hmmmmm...

...i need coffee! or something stronger

No matter what you read, you already know it doesn't work, if it did you wouldn't be here asking. Pull the small valve off the front and drive it, the combination valve is doing the same thing. If you find that after setting, the pedal is soft on the first push, then you know you need to do something different.

Joe

Easy test. Hold a string from center of front wheel cylinder to rear wheel center of cylinder. If the truck is lowered to the point that the master cylinder is below that string you probably need the valves. I was sold on the valves also back in the day when i installed disk brakes but ended taking them out and the brakes always dragged. Even when lowered 3 odd inches the MC is still above the wheel cylinders.