Brake boosters

[pottsthomas]

I did a master cylinder and related caliper work last year. When buying parts I bought a booster since it didn’t cost much and my old one was looking ratty even tho it seemed to work fine. Everything went back together with my mechanic having the usual sort of problems getting them bled. Problem is you can feel the booster working at lite pedal pressure but the harder you push the less it works. To come to stop requires a lot of force. No way you could lock the brakes. We thought it was a check valve but as I looked at the booster ad I see there’s internal check valves on some. What’s going on?

[Nut124]

What makes you think the booster is working at light pressure?

Did the brakes ever work well after the bleeding?

Is there good vacuum at the booster?

Still air in the fluid?

[pottsthomas]

At lite pedal pressure you can feel that the braking is boosted so you assume it’s working. But the harder you push the less the boost and as I said to stop is a mighty push. Been that way since the brake work. Maybe a vacuum line is tight at lite pressure but opens up when pressure increases? My question tho is about the check valves. Do some boosters have internal valving and others rely on an external valve?

[18Fiatsandcounting]

My question tho is about the check valves. Do some boosters have internal valving and others rely on an external valve?

Not that I am aware of. The check valve is usually inline in the hose that goes from the manifold to the booster. Its function is to allow vacuum to "flow" to the booster but block any significant pressure from the intake manifold. This could happen if the engine backfires through the intake, and that could damage the booster membrane if the check valve weren't there.

The easiest check is to disconnect the brake booster hose (plug the end going to the manifold) and see how the brakes feel. They should be harder, and some people prefer that. But you still should be able to stop the car although it takes more leg effort. If the car is really hard to stop, it sounds like there is something else wrong, like you're reaching the end of travel on the master cylinder before it reaches full brake line pressure to each wheel.

-Bryan

[pottsthomas]

Thanks Bryan. We will keep playing. Something is definitely amiss. I've had the car since new. I did see an ad for a booster which mentioned internal check valve which prompted the question.

[iphaul]

Hello Thomas,

Checking a brake booster is quiet easy.

1) with the engine off, push 5 times hard on the brake pedal
2) push a 6th time on the brake pedal and keep the pedal in this position
3) start the engine without pushing the accelerator
4) you should feel the brake pedal going deeper (this is the assistance from the booster)

If this works we’re going in the good direction…

5) check the vacuum line. Disconnect from the booster. With stationary running engine the vacuum is maximum and you can feel it by putting your thumb on the end of the line.
6) The one-way valve you check if flow goes only in one direction and not in the opposite.

If 4 - 5 - 6 is ok it’s still possible that the booster is not working well when hard braking.
Therefor:
7) check there is free play when pushing the pedal, i.e. the first millimeters pushing the pedal should not activate the brakes
8 ) Last step (you will need to disconnect the brake lines to do this)
Dismantle the master cylinder from the booster and check if the O-ring is present and not damaged. This O- ring is important and its function is to keep the vacuum inside th booster. If it is damaged, with maximum vacuum (at idle or deceleration) your booster is working but once the vacuum is less (throttle open), there will more leakage tan vacuum.

Best regards,
Paul