Master Cylinder, right?

[opus10583]

Hi,

Brakes are firm and effective on the road but pedal drops at lights and stop signs... Need a master cylinder rebuild, correct?

Thanks,
Mark

[124ADDHE]

Sounds like it to me, they are cheap at least

[baltobernie]

Sounds like a leaking booster to me. If you increase the RPMs, does the pedal firm up?

[opus10583]

Sounds like a leaking booster to me. If you increase the RPMs, does the pedal firm up?

I wouldn't think so, the pedal drops, the effort does not rise.

[opus10583]

Sounds like it to me, they are cheap at least

Indeed, "why not?" Thanks!

[perthling]

For rebuild, read replacement. New units are not particularly expensive.

Don't forget to check your hoses while you have the system open. It's a pain to have to bleed the brakes once - let alone twice!

[opus10583]

For rebuild, read replacement. New units are not particularly expensive.

Don't forget to check your hoses while you have the system open. It's a pain to have to bleed the brakes once - let alone twice!

Stainless-braided hoses in hand, didn't think to order the master cylinder.

[opus10583]

Sounds like a leaking booster to me. If you increase the RPMs, does the pedal firm up?

I'll check that out today, thanks!

[majicwrench]

Booster won't cause that. Either fluid leaking past seals in Master cylinder OR a leaky hose.
Usually, when you have a pedal fade at stops it is the MC. However, personally, I have been fooled, by leaky hose. The pedal is fine stopping, but if you hold the pedal, like at stops, enough fluid leaks out that the pedal fades. So if no leaks, replace MC. And I would just replace it, they are cheap.
Keith

[TulsaSpider]

I've developed the same issue. The brakes are OK when running (good vacuum) at the stop light they go to the floor and the idle goes down until I let off the brake, then it will go back up. If I pump the brakes the idle will increase a bit too. I think I have a small leak in my booster diaphragm. lots of vacuum, seems good, low vacuum loose pedal, idle drops. I have a new MC and hoses.

[manoa matt]

The brake booster assists the master cylinder via the vacume acting on the diaphragm. At a stop with the engine at idle speed, there is a high vacume in the intake manifold, therefore the pedal will feel soft. During normal driving vacumn will be lower and you will feel a firmer pedal. Initial road test reports from the late 60's and early 70's describe the car as having over boosted brakes.

[TulsaSpider]

My idle drops to the point of the car dies or would if I let it. if the engine is idling as I come to a stop I have little braking power so it's kind of a safety issue too. I do have bigger cams, Mark's street cams. Is there some kind of test I can do on the booster itself? blow air in it to see if it leaks?

[baltobernie]

First check the air hose from the intake manifold to the booster for cracks, or if it's so soft it can collapse under vacuum. If the hose and fittings are OK, suck up the booster and watch the gauge if it leaks?

http://www.harborfreight.com/mityvac-va ... 4AodygOdOA