Here is my issue, on, or around, December 1st I was driving my 1969 F250 and all of a sudden, while stopping, the brake pedal went to the floor. I still was able to stop, since there was still minimal braking that took place.

I have manual drum brakes. All original.

I deduced that I must have ruptured a brake line to the front brakes, since this has happened to me before, and my symptoms described above seem logical.

So, I got home, and first checked the master cylinder for fluid. The front side of the reservoir was empty. This pretty much confirmed my assumption that a front brake line let go.

Fast forward to today. I decided that I would fill up the front reservoir and pump the brake a million times to see where the fluid was squirting out of the bad brake line.

I did that and searched for the leak. None found. After pumping about 150 times, I checked the fluid in the master again. The level was EXACTLY the same as before. No fluid had leaked out.

I then assumed that my problem must be a bad seal in the master cylinder for the front portion.

I began to remove the master cylinder, and decided I would remove both brake lines from the master, and "pump" the fluid out into a container. When I cracked the fitting open to the front portion, it made a "hiss" sound like there was air escaping, and then fluid began to dribble out. Just for fun, (I don't know why) I decided I would pump the pedal a few times to see if I would get fluid to squirt out.

When I pumped the pedal, to my surprise, it was nice and hard! It felt like I had good brakes.

I tightened up the fittings on the master again, and decided to take it for a road test. As I was backing out of the driveway, I stepped on the brake and the truck immediately came to a screeching halt! Wow, I had brakes.

I went around the block and noticed that the brakes were good. Too good. Before, I used to have about half a pedal of travel before the brakes would really grab. Now I barely touch the pedal and it'll lock up the front wheels. The pedal travel is probably about 1/4 of and inch. I swear. It is so short, that it isn't even enough to close the brake light switch!

I even tried to stand on the brake pedal, pushing it to the floor as hard as I could, just to see if something would "let go." No dice. Still the same.

I thought maybe I had a wheel cylinder hung up, so I jacked up the front, and the rear. I was able to spin all for wheels without any resistance.

So my question is, what happened?

I figured, if it is air in the system, I would have spongy brakes, not super firm.

If it was a bad master, again I would have spongy brakes.

Could the proportioning valve be bad?

I'm so stumped. I like having firm brakes, but I can't drive it this way.

 

I'm not sure, but here is a couple things to might want to try to help pinpoint the problem.

Test it in some gravel and see if it is only the front brakes that are touchy--do the rear work at all?

Try disconnecting and plugging the vacuum line to the brake booster and give it a test drive--see if it makes much difference; I'm not really sure how they work, but possibly something could be jammed in there.

Kris

 

I'm having a similar problem on my '67 F100 with the manual drum brakes. I've replaced all of the wheel cylinders and bled it all carefully. The brakes are usually good, with about the right amount of travel and feel. But, now and then, the pedal will go down almost to the floor, feeling like a blown line. A fast second pump and they're back. Good for a quick thrill in traffic! But, on inspection, there's no fluid leaking out anywhere. The front chamber in the master cylinder will be low, and the rear chamber will be high (and may have overflowed a bit).

I think that it's internal to the master cylinder. Some combination of worn seals is pushing fluid from one half into the other. I'm going to replace the master cylinder next; it's the only thing left.

Is it possible that a sticking proportioning valve could cause this? It seems that fluid is being pushed from the rear circuit (front chamber) into the front circuit (rear chamber). From my understanding of the proportioning valve, I don't see how that could happen. So, I'm assuming that it's internal to the master cylinder.

The only reason I didn't replace the master cylinder when I did all of the wheels is because I've been undecided on whether to do the conversion to power drums, or maybe the power discs. I figured I'd drive it around a little to see how I liked the manual system. So far, I kind of like the manual drums. It takes some leg muscle, but I like the level of control. Since I mostly drive it very lightly loaded, I worry more about locking the rear and spinning the back end around, than I do about pure stopping power. If I start using it for more heavy hauling and towing, I might change my mind. This is a fun project truck and keeper, not an essential driver for me. One thing at a time.

 

mkoser,
First, if all is plumbed as original, the front reservoir is for the rear brakes and the rear reservoir is for the front brakes. How the dual master cylinder system works is when either the front or rear system looses it's fluid the other system will still work in a marginal manner by pushing the pedal all the way to the floor. This is a safety feature first mandated in 1967. It sounds to me like your rear brake system (cylinder, master cylinder or lines) may have a leak, likely a slow seeping, and this leak emptied you rear system reservoir causing your pedal to go to the floor. You the refilled it and bled out the air between the master cylinder and line and , "voila", you've got brakes again. This seriously warrants further investigation and keeping a close eye on your fluid level, as the leak continues/progresses, this is likely to happen again. I would look closely at the backing plates on the rear for evidence of brake fluid moisture. I would then pull the drums and check the wheel cylinders for leakage and replace as needed.
Good Luck,
Gene

 

Hello,
Thanks for the replies so far.
This truck is all original, manual drum brakes. There is no booster.

The portion that was dry was the rear portion of master, that controls the front brakes.
You are correct. When the brakes went soft, I still had rear brakes.

I havent lost any fluid since then. I am sure there is a leak some where, and I will fix that at a later point.

My issue now is the super super super short brake pedal travel, and just about instant lock up of the brakes if you merely blow on the pedal....

I have never seen this issue before.

Perhaps next weekend I'll pull all the drums and see what is going on.... I am sure that will be a super fun job.

 

Well, decided to just buy a new master cylinder and replace it. I went to Autozone, and ordered one. 17 bucks. Not bad.

Went to pull the old one off today, and realized that I had to disconnect the push rod from the pedal to get it out. I also noticed when I removed the "boot" over the push rod that it was full of brake fluid... Found my leak.

I got it out, and went to bench bleed the new one. I noticed that there was no push rod with the new cylinder. I read the directions with the new cylinder (sigh,) and it said that sometimes you need to put the cylinder in a vise and tap on the cylinder to get the push rod out. Well, an hour later, tons of swearing, and a cut finger, I have an old master cylinder with both mounting "ears" broken off, a crack in the reservoir, and a bent and mangled push rod still inside.

I can't get apart to save my life. I even tried to take the clip out, to pull the piston out, but the little "ears" are broken off of the clip, and I can't get it out to save my life.

I called Oreilly, and Napa. Both have them, WITH THE PUSH RODS included.

I called Autozone about my issue, and they are not able to locate one with the pushrod. They also won't take the old one back, because I already bench bled it and put brake fluid in it.

I also will be out a core charge, because my old master isn't good for anything anymore.

Oh boy.

MK