Hello guys I have a question about my 1990 f250 351w
The truck will stop, but the pedal goes all the way to the floor with not to much pressure. Is this the master cylinder, or break booster, or what? I did start the truck and pump the breaks for awhile and there are no apparent leaks, and I have lost no break fluid. Thanks,
Ryan

 

Use line clamps (not vise grips!) and clamp off all three hoses. Press on the pedal. If the pedal drops, you most likely have an internally leaking master cylinder. If the pedal remains hard, remove the clamps one by one. Once the pedal drops, you've found your leak. Also, when was the last time the rear brakes were adjusted? The so called "self adjusters" never work in reality, so manual intervention is needed to keep things working correctly.

 

We have never adjusted the back brakes since we got it, a year ago. How do you adjust them? Thanks for fast input!!

 

Remove the drum, spray brake cleaner on all the components to remove the dust (don't breathe it in!), and look for the star wheel. It should turn easily one way, as there is a metal tab to prevent it from (easily) turning backwards. Rotate it a few "clicks", reinstall the drum, and turn it. Do this until there is some drag when trying to turn the drum. Repeat for the other side. Don't overdo it, since the shoe material expands as it heats up. Adjust them too tight and you'll find yourself with a locked up wheel or one that's extremely hard to turn. Too loose is better than too tight.

You may need to hit the drums with a hammer to break them loose. Don't be afraid to hit them hard, it won't hurt them. They typically "pop" and slide outwards about 1/8-1/4" once they're free, then slide off. You may need to spray PB blaster around the hub to aid in sliding them off.

If you notice a front to rear pulsation (youll feel it in the seat) after adjusting, the drums are probably warped and out of round. Some auto parts stores machine drums and rotors, some shops will take walk-in drop off work too. Otherwise new drums will be in order if they exceed discard spec.

 

Does it mean anything that, while the truck is running, i can pump the breaks enough to build pressure on the brake pedal, but then quickly fades away??

 

With a pedal that drops like that, if you had a leak it would be REAL easy to find! My bet is the master cylinder took a dump.

 

OK, I have a parts truck that I can take a master cylinder out of,( I know it works) do I need to bench bleed the master cylinder coming off of the parts truck? Thanks

 

Wouldn't hurt.

 

Got the rear passenger side hub off and the inside is coated with a black "gunk." Is this normal? I figured I found my problem.

 

No, your brake cylinder is leaking, mixing with the brake dust and creating that gunk. It's either that or the axle seal leaking. I'm leaning to the 1st because it's more plausible, having the weak brake feel you're having.

 

So your thinking I either have a leak somewhere, or a bad master cylinder?

 

No, you found gunk so you definitely have a leak in the rear axle, since you lose pressure when you press the pedal, I think the brake cylinder on the passenger side rear wheel is broken. If the rear cylinder is actually broken it should be easy to spot, but the residue you found could be old. If that's so, and there where no recent leaking items there, go with the master cyl.

 

And PLEASE do not use a USED master cyl. They are under $100 for a FORD one

 

Use a small flat tip screwdriver or pick to gently pull the boots back on the wheel cylinders to check for leakage. A little dampness is normal, but if fluid runs out, they're junk. If the shoes are contaminated with brake fluid, they must be replaced. You will never be able to get any sort of fluid out of friction material.