Been going through and cleaning up a 86 F150 that's been sitting for a long time and slowly getting through all the maintenance.

I put on a new brake master cylinder and everything has been fine for about 500 miles or so.

Randomly the pedal got soft and the brake light came on. There have been no leaks anywhere and when I opened up the brake master cylinder the front (smaller) compartment was dry for the rear brakes.

I filled it up and re-bled them but it emptied out by the next day.

Looked underneath and didn't' see anything wet, rear brakes are all dry by the wheel cylinders (didn't take of brake drum)

only other thought is the fluid is getting past the master and getting sucked into the engine via the vacuum line...but that seems bizarre and the master cylinder is also brand new.

Anyone have any ideas??

 

Pull the rear drums I bet you will find where the fluid has been going it just has not made it outside where you can see it.
I would also follow the metal line front to rear to make sure it is free of leaks.

The fluid can not make it from the front to the rear of the master as the 1 is sealed from each other.
Look at the seal in the cover and the devider in the body.
Also once the level drops there is no way fluid can run "up hill" to the over side.
Dave ----

 

You have a leak, you just need to find it. If you are losing all of it in a day, you should be able to find it. The most common places for leaks are the rear wheel cylinders and the line behind the gas tank on the frame.

 

Is it possible for the fluid from the front to get into the vacuum booster ?

 

Not with out the rear dropping down first that I can see.
Dave ----

 

I've experience this but when the rear seal on the master cylinder leaks, it usually runs down the front of the booster, indicating that it's obviously leaking. It's possible that some brake fluid is making into the booster but they're not designed to be fluid tight. So, the brake fluid should (eventually) leak out.

It does seem weird that there's no obvious pools of brake fluid on the ground anywhere.

 

I've had fluid go into the booster before. I'd pull the rear drums first and check the wheel cylinders first, then carefully look over every inch of line for leaking fluid. If no leaks are found then pull the master cylinder and booster and see if the booster is filling up. As mentioned before it will probably be running down the front of the booster as well but you never know.

 

If your booster is not fluid tight then it is not air tight either.
Boosters will hold fluid to the point if you keep filing the master because it is low, chamber close to the booster, and not find a leak.
It can fill up to the point where going around corners the fluid will move and get sucked into the motor and you have a smoke screen behind you but as soon as you go straight no more smoke.

I have not had that happen to mine but on a non-Ford it has had brake fluid in it and now I need to get it rebuilt.
Thing is it has to be sent out for rebuild as you cant get it in the parts stores. 3 year only booster and the car manf. went out of buss in 1988
Dave ----

 

One more place to check. Look inside the cab, where the brake pedal pushrod passes through the firewall. Sometimes the booster leaks in such a way that fluid gets into the cab and runs down under the carpet or floormat.

Not sure a leak there would explain how you're losing fluid from only the reservoir for the rear brakes, but it is quick to rule out.

 

I can see that happening with manual brakes with no booster between master and firewall but never on a power brake set up.

Now I have seen Hydro power brake system leak oil, PS fluid, inside and under the carpet and thru the carpet.
Makes for a fine time trying to stop and steer, no fluid for either, and oil on the rug gets on your shoe and you cant hold it on the brake peddle that now need more pressure!
Dont ask how I know this
Dave ----

 

Huh, I always thought there was a rubber bladder inside the booster housing that made it airtight. Every time I've seen a master cylinder leak into a booster, it's into the housing which eventually starts dripping out. Whoever explained brake boosters to me must have had it wrong.
I've never experienced this nor heard of anyone who has. But, that's an easy way to diagnose.

 

It would take a long time and a lot of fluid to fill up a booster enough that it get sucked into the motor through the vacuum hose.
Dave ----

 

To update everyone, ended up being both rear master cylinders were leaking. They were dry on the outside and didn't even look that bad when I took off the drum..but it was definately the problem.

 

I will do it. Both rear wheel cylinders were leaking. We all know but newbies may not.

 

If it is emptying the front chamber quickly, that is the first place I would suspect if that line is as old as the truck.

My truck started out as a slow seep, requiring topping off about every 90-days and then one day it became a gusher, emptying the front chamber with one or two applications.

Under the truck was clean and dry; everywhere was clean and dry; but, I was losing fluid like crazy.

The ground under the truck was clean.

I found it by having the wife sit in the cab and pump the brakes while I was laying under there watching.

I was able to replace the line without dropping the tank.