I recently installed a new brake master cylinder with reaction valve, and brake servo with the tandem slave cylinder. The brakes bled easily and provide a decent pedal with the engine off. But as soon as the engine is started, the brake pedal rises all the way up, the brake lights come on, and the car will not budge. Turn off the engine, the brake pedal goes back down and the car rolls.
I have about 1" free play at the brake pedal. The upper pedal bar has 1/4"
space before depressing the master cylinder push rod.
Both brake units are original equipment brands from Welsh Jaguar, a reliable source that I have used for 30 years. All vacuum lines were correctly installed and confirmed based on p.178 of the Haynes manual. I also inspected a friend’s '66 E-type to double check the vacuum line connections.
Reviewing the Haynes detailed description, my guess is that the reaction valve is not proportioning the vacuum properly. My 25 yr old M/C and reaction valve were still working, so I could re-install the old M/C to confirm my suspicions.
What could be causing the “full on” “full off” vacuum to the servo booster?
Any suggestions other than reinstalling the old one to see what happens?
You’ve either mis-connected the hoses to the reaction valve, the piston in the nose of the master cylinder (the one connected to the pedal) is stuck, or you got a bad reaction valve. With engine running, foot off the pedal, you should have full manifold vacuum to BOTH sides of the booster. I think you’ll find you have vacuum only to the front side. Yours obviously thinks the pedal is being firmly depressed all the time.
Regards,
Ray L.
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Ray: Thanks for the quick reply. I’ll use my vacuum gauge to determine if I have full manifold vacuum at both sides of the booster, and proceed from there.
Ray: You were right! I had no vacuum coming from the front of the reaction valve to the back of the booster. I had a bad vacuum connection at the back of the reaction valve, so it was not getting vacuum. When I tightened the hose clamp much tighter, the vacuum was created and the brakes worked properly.
Now I have to find a better fitting vacuum hose. The connection at the back of the reaction plate is larger than the connection “T” fitting to the vacuum hose.
So I used the correct size vacuum hose for the reaction connection, but it won’t clamp down properly at the “T”. Perhaps the suppliers have a special hose for those fittings, small at one end and larger at the other.
Thanks for solving the vacuum issue for me!
Keith