My 1991 Ford F-150 loses brake pedal. Not all the time, but now and again suddenly. I can pump it up and it will be fine for a while, then it will happen again. Is this evidence that a new Master Cylinder is needed? If it is it will be the third one in this truck in 110,000 Mi. Also, does anyone have any idea about what they run installed?? It's been a while since I had it done. Bill T.

If you can pump it up and get more pedal it might just be air in the system. Give it a good once over and check for leaks. If you don't find any, give it a good bleed. If nothing's leaking and you don't get any air, it very well could be the MC.

I agree, first bleed the brakes. If it still does it then swap the master cylinder.

If you master cylinder is losing its seal one thing will be happening for sure, you will be needing brake fluid in the reservoir all the time. Second it will either start leaking right where it goes into the vacuum housing/brake booster and you will be able to see the fresh oil or it will be in the vacuum housing in which case it will be sucked into your manifold making the truck smoke like holy hell on occasion.

Replacing the master cylinder is as easy a brake job if not easier. Its definitely a do it your self shade tree mechanic job. Other than bleeding the brakes the hardest part is priming it with a pencil. A little trick if you have to replace it is put a little petroleum jelly on your fingers, yes Vasolene, and lube up those "O" rings before installing. It will increase the life a lot.

A rebuilt cylinder will cost you about $50 and a new one $75 to $100. Job takes about 1.5 hours and most of that the brake bleeding. There is a shade tree trick to not having to bleed the brakes by purging the cylinder good/priming, keeping those lines up so they don't spill, topping them off with a syringe, and then trying. If the peddle is spongy then you may still have to bleed.

Hope this helps,

Tj

I would check the system first to see if there is air in it. It could be the master cylinder. Before you replace I would check to see if it is leaking on the back side of the master into the booster. You can check to see if there is fluid in the brake booster. Take the vacuum hose and check valve out of the booster. You can stick a tie strap down the hole and see if there is fluid on the stick. Just like checking the oil. If there is fluid on the stick, you are going to have to replace the booster also.

That way you can check it before you pull the master cylinder off.

Check to see if the wheel cylinders are leaking on the drums. Those seals go bad too, check everything you can before you just go guessing and buy new parts.

Another quick item to check is the front caliper slides/hardware. On a lot of Ford products the slides will rust to the point of not letting the caliper slide freely. When this occurs it can cause the front brakes to develop too much heat and actually boil the brake fluid causing this same condition. Also check to see if the rear wheel cylinders are operating properly. If they are rusted or sludgey it can add to the same problem. If you have air present in the hydraulic system it will ALWAYS be there until the system is purged.A good test for this is to pump the pedal 8 or 10 times without the engine running. At that point the pedal should feel rock solid. Take your foot off of the pedal completely for 15 seconds and then hit the pedal again.If the pedal feels spongy or drifts close to the floor then air is the culprit.
Have you been adding any fluid?

There are zero leaks anywhere, and I haven't had to add a drop of fluid. Bill T.

Replace the master cylinder. Make sure you "bench bleed" the unit with the supplied hoses before installation. Bleed all 4 wheels until nothing but new clean fluid comes out of the bleeder screws with no air bubbles.
Note of caution. When bench bleeding the new master cylinder to not travel more than 1 inch when pushing the bleeder rod. I use a 1/4 nut driver with no socket on it. I measure 1 inch back and wrap a piece of tape around the shaft to give me my maximum travel line.
most people dont realize that when you step on the brake pedal the rod inside the power booster only travels about 1inch. If when bleeding a new unit most people will bury their push rod until it bottoms out inside the master cylinder housing. This is not good because you risk "stroking" the master cylinder piston and seals way beyond their normal rate of travel leading to premature failure of the "O" rings around the piston assembly. If you have already done 3 master cylinders in 100,000 miles than you may be a victim of this action.
Any other questions related to automotive just let me know.
The only thing I have ever done to make a living revolves around automotive technology.
Currently I manage a Firestone Auto center in Ohio.
I have been working on vehicles for over 25 years.

Another trick for bleeding the brakes, is to remove the bleeder on one wheel at a time (tried this with disc brakes, not drum yet), and just keep adding fluid to the master cylinder until you have fresh, clean, new fluid coming out, then put the bleeder valve back in. It can use a lot of fluid, but this way you're guaranteed to have completely new fluid in the entire system. Let gravity help you out.

Kenny Ohio,

Thanks much for the informative reply. "I" personally haven't done any of this work. The last master cylinder was installed by Midas a few years back. Other brake speciality places did the others. I can't remember which ones they were offhand. I'm betting it's the master cylinder based on the fact I've had so many go bad. They may have been damaged by improper installation and bleeding as you suggested. It's been a while, and I was wondering what a "ballpark" cost would be today for a new master cylinder and brake fluid renewal installed, out the door? Thanks, Bill T.

Depending on where you live a shop would probably charge $200.00 to $250.00

If you pump it up and it holds firm you most likely have air in the system. If you hold it and it still falls you probably need a master cylinder. Don;t know about Ford but GM trucks are famous for having excessive pedal travel.

If it's loosing fluid I would find the leak. If you can't find it, it is probably a wheel cylinder.

Otherwise bleed and then maybe a master cyl.

Make sure your e-brake works.

I had it done last week. New Master Cylinder, and replace 2 leaking back wheel cylinders. Flush out, and fill the entire system. $688.00 out the door at Midas. Bill T.

OUCH!

I will say the brakes on my truck are the absolute best they've ever been. I noticed immediately that much less pedal pressure is required to stop. I don't know if it is the new Master Cylinder, or a combination of everything they did, but there is a most definite improvement in the whole system. In that regard it was worth it. Bill T.