Thread: Brake booster
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Old 26th June 2016, 10:33
Mike Mike is offline
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Originally Posted by mcramsay View Post
I have the Marlin supplied dual brake booster, coupled up to a Sierra master cylinder. With the engine off pedal goes rock hard with only a little bit of force on the pedal. However with the engine on the booster is doing its job so well the pedal goes to around 3/4 travel before you actually get any feed back. It's almost like the booster is working too well

I know this is a widely covered topic on her so how do others find it? Have I got something wrong on my set up? It's hard to tell how much braking I'm doing as the car is only creeping forward and back on the driveway!

Due to the design of the dual servo, there is more pedal travel with it than without, as the first 10-15mm of travel is soaked up in a compression spring movement which is required to allow the vacuum/external air pressure valve seal to seat correctly.
I think we have all found there is more pedal travel before the brakes start to bite than compared to no servo, but then the brakes will bring the car to a safe stop with ease. Without a servo, or indeed with the under powered Metro servo this was just not the case: they were just unsafe in today's traffic

Wait until you get onto the road (private one of course!) to test the travel and their efficiency. If they don't stop you from 70mph with half pedal travel, you have an issue that I do not experience.

Whatever you do though, do not try to over extend the adjuster screw to over compensate for this additional travel: this only results in the servo being slightly activated with the pedal at rest, letting atmospheric pressure create pressure on the diaphragm, which can (has) lead to severe binding of the brakes.

Make sure you fit a pedal return spring as well to ensure the vacuum seal is properly closed.
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