I have 70 F100. Anyways, it has been getting hard to stop. Then today, i was doing about 60mph and had to slam the brakes, the truck would not stop! I narrowly avoided crashing by pumping the pedal and if finally stopped.

I assume this is the master cylinder, right? I notice a small amount of fluid on the outside that seeped out?

My second question: is the master cylinder hard to replace? I know you need two guys to bleed the brakes, but i haven't done that before.

Thanks in advance for the help,

Mike

 

I have bled brakes by myself before. I believe there is a "one man brake bleeding kit" available at auto parts stores now. Remember to "bench bleed" the master cylinder before you install it. You can also buy a "bench bleeder kit" at auto parts stores.

 

Oh, yes, "pumping" the brakes and having the master cylinder go from not working well to working better is a sign of a wearing or worn out master cylinder. The brake fluid you speak of, is it kinda running out around the firewall?

 

No, it is running on the front of the m/c itself.

 

When the plunger inside of the master cylinder wears the brake pedal gets super easy to push and it will go to the floor if you do not pump the brakes. That needs to be replaced before it fails completely.

The running fluid, front, do you mean it is running down the radiator end of the master cylinder, from around the cap and cap seal area?

I have had master cylinders leak into the passenger compartment on the inside of the firewall. I have had them leak down the outside of the firewall. Just recently I had one that I haven't removed the fluid cap on leak because of the rust build-up on the cap seal from the top edge of the master cylinder, from sitting a long time (I cleaned the rubber seal good and removed the small rough spots off of it and it stopped).

 

If and when you removed the master cylinder cover was one of the reserviors low on fluid? IF SO, take the time to inspect the rear of your brake drums (what the drums stop at, or what they call the backing plates) and the back sides of the tires for what will look like wet trails of oily residue in streaks down the tires which could also be a tell-tale sign of a leaking wheel cylinder.

If one reservior on the master cylinder goes dry the pedal will get mushy too.

 

okay, will do. Thanks for all of your help.

 

Mike, Sounds like you are facing some major work and would a great time to upgrade the truck to power disc brakes.

One of the better things that I did for my truck.

John

 

Sorry to sound stupid but what is "bench bleeding"?

 

Directions should come in the package with the cylinder, but it is a method for removing air from the cylinder before it is installed.

John

 

Wear some safety glasses or something to protect your eyes from the possibility of splashing brake fluid into your face and eyes if something goes wrong while doing this!!

Some master cylinders come with some plastic fittings that go into the threaded female fittings that your lines would screw into, a clip to hold the lines to the dividing wall between the reserviors and some small rubber hoses that get slid onto the nipples of the plastic fittings. If not, the bench bleeding kit comes with these things too.

You place the master cylinder into a vise (in my case I hold the master cylinder because I don't have a workshop or a vise), install the fittings into the new / rebuilt master cylinder, put the hose holder on the divider between the reserviors and attach the hoses to the fittings and the reservior clip.

Fill the two reserviors with the appropriate brake fluid (ONLY about 1/2 to 3/4 full) and make sure the two tube ends are sitting beneath the surface of the brake fluid. Try to get the tubes to sit close to the bottom of the reserviors if possible so they don't run dry during the next step.

I use a good phillips screwdriver (others have other ways of doing this part of the job) and while the master cylinder is held tight and still you need to push the plunger on the master cylinder (the thing the push rod that goes to the brake pedal pushes on to stop the truck) all the way in a few times to get the brake fluid started through the master cylinder's chambers. You will see air bubbles coming from both tubes when you do this. You will also see the brake fluid level go down. You are pushing the air out of the chambers in the master cylinder and replacing it with brake fluid.

Check the fluid level and refill as necessary.

Continue pumping the plunger until you see NO bubbles coming from the rubber tubes while monitoring the fluid level in the reserviors. YOU MUST see air bubbles from BOTH tubes when you do this procedure at first and during this procedure and if you don't you aren't pushing the plunger into the master cylinder far enough. Don't be "macho man" when you are doing this either, use steady even pressure because you can push fluid up into a small fountain-like bubble on the surface of the brake fluid and have it push out over the top edge of the reserviors and get it all over your work area (like I did the first time I tried my hand at bench bleeding) or you might get it all over you.

When you have done this for a short while, and the reserviors haven't run dry and the tubes didn't come out of the fluid you should see no more bubbles coming from the tubes.

When you see the bubbles stop your master cylinder is bench bled.

A lot of times I have been able to install the master cylinder on my vehicle with the bench bleeding kit still in place on the master cylinder. I haven't done one on my 1968 F100, yet so I don't know if this is possible on these trucks. I remove the bench bleeding kit fittings when I am ready to reinstall the brake lines to the master cylinder (one at a time). I don't know if this is necessary but I like doing it this way, I lose less fluid from the new master cylinder's fittings this way (I believe).

 

well, got it all done. Bench bled the master and after i put it on i just bled the system by loosening the fittings on the m/c, pumped brakes held, and tightened. Worked like a charm, brakes are great now. But seriously, thanks for all of your help l1k2gadd.

 

You're welcome.

Some will say you need to bleed the entire brake system after you replace the master cylinder.

I've done the same thing you did after replacing a master cylinder and didn't have any issues that I can recall on several old Mustangs and other car I have owned in the past. Check on a dry day and then on a wet day on a short, hard stop with no other vehicles on the road (maybe on a street close to home or a back road) and make sure you don't lock any wheels due to a possible imbalance in the fluid system (due to an air bubble in one of the lines making one brake not work evenly with the others).

I bought my 1968 F-100 about 5 months or so ago. I just did the front brakes on my 1968 F-100 yesterday (I had one grabbing and sticking bad). When I would hit the brakes hard, the steering wheel would pull to the left real hard making the truck want to cross into oncoming traffic. I replaced the brake shoes, wheel cylinders, brake hoses (made in China from Napa), all-in one new spring kits and new self-adjuster cables. People say these trucks don't stop worth a dime, or on a dime with drum brakes. I was worried with all I have heard BUT now I can't wait to replace the rear brakes (the same way with all of the hardware as I did with the front brakes). My truck seems to do great with drum brakes, now that the problem parts have been replaced!

Take care! If you decide to bleed the brakes look into the one-man brake bleeding kit if you don't have anyone to help you with bleeding the system.

 

where do you get a new master cylinder? or what is the best repair kit for this failing plunger that causes you to have to "pump up" the brakes?

 

i just got mine from cskauto.com