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My '66 F250 with fairly stock setup including manual drum brakes is displaying what I think are indications that my master brake cylinder is bad. Recently the brakes get spongy and sometimes when I hold my foot on the brake pedal it will sink slowly toward the floor.
Have others had similar symptoms? Was it the Master Cylinder that needed fixed? Did you rebuild it or replace it?
I would check your entire system for leaks. Brake fluid can go bad and cause some of the same symptoms. It's the other pedal that should go to the floor. Good luck.
hahahahaha yep well that happens too sometimes .. There doesn't seem to be any leaks that I can find - checked around the wheels and on the garage floor which is pretty clean. I also have not had to refill the reservoir since I first got her about 1 year ago... hmmmmmmmm
It sounds like air might have gotten into the system. I'd start with a good bleed of the system with some fresh fluid if you haven't done so already.
My favorite tool for the job is a pressure bleeder. You remove any residual air pressure and unscrew the T-handle top. Pour in up to a gallon of fresh fluid into the inner bladder and put the T-handle top back on. Then pressurize the tank with about 15 psi, which presses on the inner bladder.
Next remove the master cylinder cap and attach one of the adapters with an appropriate rubber gasket (lower one in box for the large round masters). Using a container, turn on the flow using the valve at the end of the hose until fluid runs to remove any air from the hose and tank, then turn off the valve. Hook the tank hose to the adapter and turn the flow valve back on.
Then just go to each wheel cylinder bleed valve, farthest away first, and work forward bleeding each one until it looks like fresh brake fluid. I use a clear tube, with about 1/4" or 5/16" or so internal diameter, hooked to the bleeder valve and going into a container so it doesn't go everywhere. Back passengers, back drivers, front passengers, front drivers.
The tank automatically refills the master as you bleed and has up to a gallon so you don't generally have to stop and refill. Great for doing it alone. And really reduces the air in the system to a very low level. No draw back into the system and no worry about running out of fluid if filling the master manually.
It's one of those tools you may not use a ton but it makes the job much more enjoyable for me.
Or a helper pumping the brake pedal method. Or a vacuum pump can help sometimes, too, but I find that less effective than the pressure tank for a good bleed.
I just used one of the kits from Phoenix Systems on my 54 Ford.
It worked exactly as they claimed it would and my brakes are excellent now. It's kind of a different approach (and probably a more logical one) to bleeding brakes.
With this system, you pump fluid up from the caliper to the master cylinder. This way, the air bubbles aren't fighting their inclination to rise up. You're working with physics, not against it.
It's truly an easy one-man system. It took one of the nastier chores of car maintenance and actually made it kind of fun.
If the MC was leaking, you'd probably notice brake fluid on the floor mat. There are MC rebuild kits available, and new manufacture available, even so. Then start looking at hoses, new wheel cylinders, a little bit of everything cost, heck me 600 bucks for the old gal but she stops good now. Kinda like that.
If your NOT having to replace brake fluid in the MS and your peddle start slowly going down while you apply pressure - than its your MS that needs replacing. If in fact it was leaking ANYWHERE in the system, (at the wheels or on the floor board) the fluid would go down in the MS. What is most likely happening is the cup seals in the MS are bad and fluid is leaking past them causing the brake peddle to go down when applying pressure to it. Simple fix - replace the MS. Bench bleed the new MS. They sell kits for that.
If your NOT having to replace brake fluid in the MS and your peddle start slowly going down while you apply pressure - than its your MS that needs replacing. If in fact it was leaking ANYWHERE in the system, (at the wheels or on the floor board) the fluid would go down in the MS. What is most likely happening is the cup seals in the MS are bad and fluid is leaking past them causing the brake peddle to go down when applying pressure to it. Simple fix - replace the MS. Bench bleed the new MS. They sell kits for that.
This is exactly the answer I would give. Its an inexpensive part to pick up and fairly simple to replace.
Replacing the master cylinder takes about 90 minutes providing you have everything on hand you may need, none of your bleeders at the wheels are rusted solid, and you have someone to help you out when bleeding.
This may also be an excellent time to change to a dual reservoir master if you dont care of its original stock in appearance. It will require a little bit of extra brake line to be plumbed in, and depending on what master you use, you may need to change the way your brakelights light up, but overall its much safer.
To qualify my statement, I have diagnosed and repaired brake systems ranging from motorcycles to antiques cars and trucks, simple systems, Anti-lock brakes, you get the point. If you are 100% certain there are not other weeps, seeps, or leaks on the system and you have not had to add any fluid, I agree completely with the earlier post that the master cylinder is by-passing and allowing the pedal to fade. If you have not inspected all the wheels (pulled all the drums off) and adjusted the brakes yet, I highly recommend you do that before spending any money on parts. Once you are certain all is correct including adjustment, check the pedal again. If it fades away, try pumping it gently to see if the pedal comes back to the top and then fades away again under steady pressure. If it does, then go buy a new master cylinder. Bench bleed before installing as recommended by previous poster. Pressure bleeder systems are good if you can obtain one but you can bleed the system without one if need be. Good luck.
It sounds like air might have gotten into the system. I'd start with a good bleed of the system with some fresh fluid if you haven't done so already.
My favorite tool for the job is a pressure bleeder. You remove any residual air pressure and unscrew the T-handle top. Pour in up to a gallon of fresh fluid into the inner bladder and put the T-handle top back on. Then pressurize the tank with about 15 psi, which presses on the inner bladder.
Next remove the master cylinder cap and attach one of the adapters with an appropriate rubber gasket (lower one in box for the large round masters). Using a container, turn on the flow using the valve at the end of the hose until fluid runs to remove any air from the hose and tank, then turn off the valve. Hook the tank hose to the adapter and turn the flow valve back on.
Then just go to each wheel cylinder bleed valve, farthest away first, and work forward bleeding each one until it looks like fresh brake fluid. I use a clear tube, with about 1/4" or 5/16" or so internal diameter, hooked to the bleeder valve and going into a container so it doesn't go everywhere. Back passengers, back drivers, front passengers, front drivers.
The tank automatically refills the master as you bleed and has up to a gallon so you don't generally have to stop and refill. Great for doing it alone. And really reduces the air in the system to a very low level. No draw back into the system and no worry about running out of fluid if filling the master manually.
It's one of those tools you may not use a ton but it makes the job much more enjoyable for me.
Or a helper pumping the brake pedal method. Or a vacuum pump can help sometimes, too, but I find that less effective than the pressure tank for a good bleed.
05-22-2014, 03:51 PM
