Ive got a 1971 Ford F250 Camper Special with stock disc brakes in front and stock drums in the rear. When I press on the brake pedal when the truck is off, the pedal is firm, but the second i turn it on the brake pedal sinks to the floor. If i pump the pedal it doesn't get any firmer. Also when I brake really hard the truck will sometimes die or the idle will go super low. When i stop the heavy braking the idle will go back to normal. Any ideas???? I am really stumped on this one.

 

You mentioned having front discs but you didn't mention if you have a power brake booster (?).

Given your description of the symptoms though, I would say you have a brake booster and I would say it's bad and needs replacing.

 

Hey Ultra that sounds reasonable to me, tho just wanting to learn here - the pedal should not go to the floor under any circumstance should it, i.e. power brakes or no? I dunno.

Usually that points to a bad M/C. Maybe any M/C should be replaced/rebuilt on general principles at this point anyway is what I'm thinkin'. The rest of the symptoms point to some vacuum issues for sure.

 

Manual brakes would have no connection to the engine and therefore, would have no bearing on the engine whether the engine was running or not.

A vacuum brake booster, on the other hand, would have a direct bearing on the engine and it would cause the engine to stumble or idle down (loss of vacuum) if the vacuum valve in the booster is not sealing off or, if the atmospheric valve isn't sealing off, it would try to apply the the brakes.

There could also be a problem with the MC --internally bypassing or, has leaked out the back and over into the booster. If that's the case, both booster and MC will have to be replaced.

 

Thanks Ultra! I have been reading others peoples issues and they all point it to being the brake booster. Its has vaccum booster fyi

 

Well again what I'm getting at, I agree with the vacuum booster part of your diagnosis. Ultra. However comma,

Under no circumstances will a pedal "go to the floor" without a serious fault in something other than the booster, right? Assuming there's no brake line or cylinder leakage, it's always going to be a M/C fault yes?

Oh, and welcome to FTE Herbster!

 

Make sure to check the back end of the MC when you unbolt it from the booster. IF it's wet with brake fluid, then you know the MC is leaking and definitely needs replacing.

You didn't mention (prior to this event) if the brakes would initially hold pressure but the pedal would drift towards the floor, after being on the brake pedal for a while (?).

 

Since we don't (yet) know the full details of what the OP's brake problems are, I was just trying to lead up to what may be the problem and what may have caused a faulty booster --after we know what the OP finds.

To go on and jump ahead before knowing what the OP finds, I think possibly the MC was going out --leaking out the back and over into the booster. The fluid finally broke the booster diaphragm down. This created the recent problem with the engine's reaction when pressing on the brake pedal. But, it's just a theory until the OP knows more from his findings and reports back with what those findings are.

 

Also I dont know if this means anything, but the brake reservoir keeps on leaking, Its very tightly on there but for some reason i always see brake fluid on the side.

 

This is a 71 right? Jeez Louise, a new MC might be $50, seems to me after 45 years it's time, we done got our money's worth outta that thing ya know? In fact if you buy a new one it may come as a set - along with the booster. It just makes sense when it comes to brakes. New wheel cylinders, hoses, etc, it all needs going over. If you live in the rustbelt new hardline is advisable, probably even if you don't.

 

Thats probably the best idea Tedster. The previous owner replaced a lot of things under the hood but im not sure what they were, so ill ask tomorrow.

 

1968/72 F250 2WD's and F350's were available with optional dual piston caliper power front disc brakes.

If the pedal is dropping to the floor, the master cylinder is leaking into the booster.

 

2X Ultra. I also think the M/C started the problem and leaked in to the booster creating the 2nd problem. In the aftermarket world you can buy the M/C and booster together in one box. Called a "combo unit" I believe. It is my suggestion you buy that and solve all brake issues. Don't forget OP, BRAKES ARE YOUR LIFE. And maybe other's.