Power booster problems

[1293kyle]

My 71 f100 has power drum brakes. I have somewhat good pressure on the brakes when the trucks off ,but the second I turn it on i loose all pressure. I have a feeling this is the brake booster ,but it may be air in the lines. I've bled the master cylinder twice and the brake lines over 8 times. If you have any ideas for the the power booster that would be appreciated ,thanks.

[Mach428]

You mention that you have good pressure on the brakes with the engine off but do you mean you have a firm pedal? Are the brakes actually engaging with the engine off?

The reason I ask is that your description sounds like a bad booster. I don't think it's air in the lines with the way you've described your bleeding. You also didn't mention of you replaced either or both the MC & booster.

It could be that they are not a good match. I've run into this to the point that now, I try to buy both together.

[1293kyle]

The power booster are cylinder are new from advanced auto parts. also the brakes still stop the truck but barely and you have to go as far as you could possibly go down on the pedal.

[Mach428]

But were they sold as an assembled set? Keep in mind these are mass rebuilt and it sounds like you have the wrong combination.

[ultraranger]

The engineers that designed the brakes for a particular vehicle/application design the parts to work together for the particular vehicle but, the MC doesn't necessarily have to be 'matched' to any type of booster for both of them to do their job. The booster is just a force multiplier. It doesn't know what MC you are using or even what type of brakes it's actuating (drum/drum, discs/drum or discs/discs).

You just have to pay attention to bore diameters and the booster's output rod engagement length to the MC.

This didn't come as a matched booster and MC from the factory for a '69 F100. --1975 Ford F350 brake booster and a 1995 Ford Explorer MC.





If all the air has been bled from the system, the booster is likely at fault.

[ultraranger]

Deleted (double posted somehow).

[Mach428]

You are correct Ultraranger. My point in saying this is that I've seen a lot of MC that do not match the booster output rod and some of the rods are non-adjustable.

[ultraranger]

You are correct Ultraranger. My point in saying this is that I've seen a lot of MC that do not match the booster output rod and some of the rods are non-adjustable.

It may be that some of the Bump/Dent boosters don't have an adjustable out put rod but so far, I haven't come across any.

I have encountered 2.3L Mustang brake boosters (--single diaphragm for an early model Mustang swap) that didn't have adjustable output rods. All the 5.0L Fox Mustangs (dual-diaphragm) have adjustable rods.

Most all Ford vacuum boosters I've dealt with required an output rod adjustment from .995"-.998" to engage the depth of the primary piston of the MC.

As with any installation, the MC rod bore depth to booster output rod length should be checked BEFORE bolting the MC to the booster.

[1293kyle]

We've took off the mc from the booster to shorten the piston in the pb for experimental purposes and I noticed some dry brake fluid on bottom (right below the piston). I am guessing that a seal is bad or something and it was leaking brake fluid inside the pb.

[ultraranger]

Brake fluid leaking out the back of a MC, into a vacuum brake booster, will damage the booster diaphragm.

[rbeck204]

A bad vacuum modulator on an automatic transmission, or a sticking choke can also damage a booster. Don't ask me how I know. FYI
Rich