In my 78 Ford F100 i've noticed i have to really pump the brakes to get any pressure to stop it. If i push on them regularly it goes to the floor. What is the primary reason be for this? Is it the valves under the brake reservoir, master cylinder, or possibly the proportional valve?

Thanks

 

Sounds like the master cylinder may be giving up the ghost, do you notice a decreasing fluid level inside the reservoir and/or fluid leaking out the rear?

 

It could be a lot of things. I recommend looking at the fluid level close and seeing if it goes down. It sounds like a leak to me. The master cylinder is the easiest thing to replace and cheap too, but you might have a bad slave cylinder in the back, a leak in a line somewhere, a bad caliper, or something. Look high and low for leaks.

 

Thanks guys, i just noticed the larger reservoir was a little low, but i've noticed over the last couple of years this is not shocking for this truck. Typically i have to refill both the small and large reservoir. while troubleshooting today the small reservoir is completely full and the larger is low, is that a key to something?

Thanks again

 

If you are losing quantities you have a leak somewhere...easy way to check is look at the back side of the wheel if it looks wet it most likely a leak...check your lines, also check around where your master cylinder is..and or inside the cab....its a closed system so you shouldnt have any leaks...unless you changed your brakes recently you shouldnt have any fluid loss.
AUTO REPAIR HELP - THE MASTER CYLINDER

Master cylinders used with disc/drum combination brake systems will use a large reservoir for the disc brake portion of the system and a small reservoir for the drum brake portion.

 

Alright a bit more troubleshooting and i've noticed around the top of the master cylinder it appears to leak around that seal. That's probably why i lose fluid periodically and need to refill them both. While driving the truck i can pump the breaks multiple times and get great break pressure and it will kick the brake light off. If i don't pump the brakes it will go to the floor not stopping the truck as well, and the red brake light comes on until i start pumping the brakes again. Once again after pumping the brakes i get good pressure and the light goes off. Does this help push me towards a particular problem?

 

Thanks wilcam, that is a nice article and makes me wonder if my problem is one of the pistons in my master cylinder is bad. maybe the fact that i pump the brakes i get it to respond, or possibly a bad seal. still troubleshooting the system...any thoughts or recommendations is greatly appreciated.

 

No fluid loss is acceptable. You should never have to top off your brake fluid. If the smaller (rear brake) reservoir fluid level changes, you have a leak that needs attention. The larger (front brake) reservoir fluid level will lower as the brake pads wear. Don't top off the reservoir unless the level becomes excessively low; doing so will cause fluid to overflow when the pads are replaced and the pistons are retracted.

If you're sure that you're losing fluid at the master cylinder and not elsewhere in the system, rebuild or replace the master cylinder.

 

Thanks everyone, i'll again for leaks or anything that looks out of the ordinary. If i dont see anything i'll go after the master cylinder.

 

Sometimes a leaking master cylinder won't reveal itself with the tell-tale signs of brake fluid running down the booster, often times the fluid will be sucked directly into the booster, pull the boosters vacuum hose and check for brake fluid coating the inside of the hose.

 

I'd pull the rear drums to see if the wheel cylinders are leaking and check that the self adjusters are working. If the rear shoes are grossly out of adjustment, you would get the light coming on until you build up pressure in the rear lines.

 

Ok, so I've went out and bled the brakes. No change. I can still pump the brakes and get really good pressure and keep it until I let off the brake. Once I let off the brake and go to press it again, it's if I have no brakes. I looked everywhere for leaks, and I didn't see any evidence at all, that I have a leak. Do you believe it is the rear shoes horribly out of adjustment, and that this can all of a sudden happen? Do I aim at the master cylinder being the problem now? Do I attempt a rebuild on it, or replace it? Thank you all for the continued support and troubleshooting tips.

 

I would adjust your rear brakes and check the operation again.

 

If the master cylinder is leaking internally you won't find a leak, it's bypassing the piston seals inside the master cylinder and no external leaks would be noted.

 

Did this happen suddenly? Don't think you mentioned that.

-Is your parking brake working correctly? If not, your rear shoes are possibly out of adjustment.
-You have a hard pedal when pumped up so I doubt there is air in the lines. Plus you've ruled that out by bleeding the lines.
-It could be your master leaking internally, but I think you'd lose pressure gradually as you push on the brake pedal. One way to check is to remove the brake lines from the master and plug the inlets. You should now have a very hard pedal. If it leaks down slowly then you have a leaking master. Another thing is to check the linkage under the dash to the master. Everything connected and straight?
-Slightly low fluid in the disc brake side is normal as the pads wear. Nothing to worry about there. Doesn't sound like you're losing fluid.
-Run out on the discs can push the pads back to where you get low pedal, but your case seems extreme. Something to check though. Do you get vibration when you brake?
-Weak, bulging soft brake lines would cause a soft pedal, but again something to check.
-If it happened suddenly, it could be something amiss in the rear brake shoe linkage. That's where I would start, I think. Pull the drums, make sure everything is connected properly, maybe clean and lube the adjusters. Then manually adjust the shoes through the backing plate to just where you have a little drag. Try the brakes then.