The Ford Barn

27 Roadster · 10-27-2019, 03:41 PM

After a panic-stop my master cylinder no longer built normal pressure. With the master-cylinder reservoir cap removed I could see a jet of brake fluid escaping from the bypass port back into the reservoir. I assumed the piston cup had failed so I bought a new master cylinder. The new cylinder bleed nicely on the bench, but upon installation the new cylinder does exactly the same thing! Even after bleeding the brakes, the brake fluid escapes out the bypass port back into the reservoir rather than building pressure in the lines. It seems highly improbable that a new master cylinder would fail in precisely the same mode as the old one. Can anything else in the system cause this?

Fordestes · 10-27-2019, 05:51 PM

When you apply the brakes there will be a small spurt of fluid momentarily from the master cylinder compensating port , if you get a steady stream and soft pedal while fluid is collecting back in the reservoir while the brakes are applied you have a master cylinder problem.

Mart · 10-27-2019, 06:32 PM

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During the panic stop you may have done something that has effectively shortened the push rod length. Impossible to say what that may be. You need to minimise the travel during which that squirt is taking place. You may not be happy to mess with the cylinder, but this is what I have done in the past.
I fitted a washer underneath the circlip that holds the piston in. This held the piston slightly further down the bore and cut down on the travel (lost motion) before the recup port is closed. This gives a smaller and almost momentary squirt rather than a long strong squirt.
Then adjust the push rod length to minimise free play. You will then get the most efficient transfer of pedal movement into master cylinder effective travel.

Remember, all the time the recup port is open (squirting) you are not developing any pressure.

Minimise the squirt, but bear in mind that it must take place. No squirt means the recup port is blocked and that leaves to other problems.

What caused the change in performance on you system, I'm afraid, is a question only you might be able to answer.

10-27-2019, 03:41 PM	#1
27 Roadster Junior Member Join Date: Mar 2018 Posts: 2	39 Brake Master Cylinder Bypassing After a panic-stop my master cylinder no longer built normal pressure. With the master-cylinder reservoir cap removed I could see a jet of brake fluid escaping from the bypass port back into the reservoir. I assumed the piston cup had failed so I bought a new master cylinder. The new cylinder bleed nicely on the bench, but upon installation the new cylinder does exactly the same thing! Even after bleeding the brakes, the brake fluid escapes out the bypass port back into the reservoir rather than building pressure in the lines. It seems highly improbable that a new master cylinder would fail in precisely the same mode as the old one. Can anything else in the system cause this?

10-27-2019, 05:51 PM	#2
Fordestes Senior Member Join Date: May 2010 Posts: 886	Re: 39 Brake Master Cylinder Bypassing When you apply the brakes there will be a small spurt of fluid momentarily from the master cylinder compensating port , if you get a steady stream and soft pedal while fluid is collecting back in the reservoir while the brakes are applied you have a master cylinder problem.

10-27-2019, 06:32 PM	#3
Mart Senior Member Join Date: May 2010 Location: Solihull, England. Posts: 8,754	Re: 39 Brake Master Cylinder Bypassing Sponsored Links (Register now to hide all advertisements) During the panic stop you may have done something that has effectively shortened the push rod length. Impossible to say what that may be. You need to minimise the travel during which that squirt is taking place. You may not be happy to mess with the cylinder, but this is what I have done in the past. I fitted a washer underneath the circlip that holds the piston in. This held the piston slightly further down the bore and cut down on the travel (lost motion) before the recup port is closed. This gives a smaller and almost momentary squirt rather than a long strong squirt. Then adjust the push rod length to minimise free play. You will then get the most efficient transfer of pedal movement into master cylinder effective travel. Remember, all the time the recup port is open (squirting) you are not developing any pressure. Minimise the squirt, but bear in mind that it must take place. No squirt means the recup port is blocked and that leaves to other problems. What caused the change in performance on you system, I'm afraid, is a question only you might be able to answer.