My brakes are driving me insane. I just swapped out the master cylinder on our '57 Ford wagon. The old one was leaking from the joint 'between the servo and the master cylinder (it has a later set up with granada discs, I do not know the origin of the servo m/c) I bought a "brand new not remanufactured"! raybestos cylinder identical to the one I took off. I bench bled the master cylinder and then bled the brakes. They worked good, then the next day whilst reversing out the pedal went right to the floor, I don't mean it was soft,I mean right to the boards and I had to stop the car with the parking brake. I removed it and checked it out on the bench, bench bled it again then power bled the car. Ok for a couple of days, good pedal. Then it done it again,It only happens at slow speeds. Any ideas welcome!
There should be a seal between the servo (booster) and the master cylinder. If not you may want to check for fluid inside the servo and if there is it needs to be removed and a seal needs to be installed. If you can take a picture of the system we may be able to identify it. It also sounds as if you have a leak in the system. Maybe a bad hose or connection. Check everything for signs of fluid.
It shouldn't go to the floor unless you have a real bad leak (in which case the fluid would be low in the resevoir), or you did a really really bad job of bleeding it (even then it should be able to "pump up" a little to regain some braking). With the car sitting and running, hold the brake down for a minute or more. If it holds at first, then slowy pushes towards the floor, fluid is getting past the piston in the MC, and it's bad. I stopped buying rebuilt MCs after getting 4 straight bad ones. Apparently when they bore out the hole, they often times don't use a larger piston, so you get fluid by pass (according to a well respected parts guy at the store I go to). The booster (servo) I think should have around 14-16 lbs of pressure (maybe someone can confirm, going on foggy memory here). If it's less, it could be bad too.
Thanks for the suggestions. I just took it all out again. No fluid in the booster and no apparent leaks under the car, checked every joint. The only thing I noticed was the 9/16 x 18 adaptor that was in the old master bottomed out on the hex before the flare seated, It's hard to tell if it was leaking fluid from there. I reduced the hex in the lathe so it will seat. Will try it again. Also checked out the m/c on the bench again with both ports plugged, seems fine. I took a picture of the booster, any ideas what vehicle it's off? We don't have the parts places you have there and it all has to be done on the phone... Thanks again. The 4 Mounting studs are M8 and are 3 3/8" c/c The two M/C studs are M10 The booster is 8" overall diameter at it's widest.
Check with the part store and have him give you the application for the "rebuilt" master cylinder you bought; then have him look up the Wagner part number. You can find out if the master cylinder had built in residual valves or not by call Wagner @ 800-325-8886, they are pretty nice there and if you ask them nice they will tell you if it has a built in residual valve(s). If you are using this on a drum brake application you will need a 10 residual valve(s) in line to help seal the cup seals in the wheel cylinders, otherwise they will leak. This might or might not be your problem. For additional information on residual valves go to: http://www.hotrodheaven.com/tech/brakes/brakes7.htm#Residual Valves Hope this helps u. dso
real quick leason on how to rebuild a master cylinder. ready? step 1: throw it in the trash. step 2: buy a new one.
Purely aftermarket booster. Similar to Ford Mustang II and Chevy Chevette, In fact I belive they are a combination of the two. Its funny to hear the hatred of rebuilt master cylinders. I use remanufactured master cylinders with my brake pedal kits. I have a recitivism rate under 8% for the 10+ years I have been doing brake kits. I have it on good athourity that my competitors who manufacture their own Master cylinders have a rejection rate nearing 25% ! Most of the problems that arise with new cylinders are stupid things like eclusions in the casting or machining burrs in the bore that tear up the pison cups. I myself have pulled brand new Bendix and Wagner masters out of the box and had problems with them, rare but it has happened. As for the improper fitting adapter, it is very possible this is the source of your problem. I would seal the fitting with some RED Loctite on a very dry fitting. In a few instances the fittings will seal well enough under pressure to not leak fluid but when you release the pedal air can escape into the system. The more you pump the more air gets into the system and the worse your pedal gets.
I had a problem last year that was very similar. Next time it happens, just try re-bleeding the rear brakes and see if fixes it. If it does, then you are getting air into the rear circuit only. If there are no fluid leaks then you could be sucking air past the rear wheel cylinder seals when you let off the brakes. Either a disc/disc master cylinder with no external (10lb) residual valve, or a disc/drum master cylinder with a faulty internal residual valve would do this. If you are not sure if you have a disc/disc master cylinder or not, try gently pushing a paper clip into the master cylinder outlet for the rear brakes. A disc/drum cylinder will have a rubber residual valve sitting behind the flare seat which you will be able to feel. If the paper clip goes in without resistance, it is a disc/disc cylinder and would need an external 10lb residual valve.
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