The 'E' Type Forum

My brakes have been like wood for a while and I have to really stamp down hard to get any good stopping.
I’ve done the standard servo test of; before starting the engine, pushing the brake pedal down 8-10 times, hold it down then start the engine to see if it sunk down further but there is no change at all.
I popped the reaction valve out of the master cylinder, cleaned and red greased it before re-fitting. One thing to note here was that there was about a teaspoonful of yellowy sludge in the plastic diaphragm cover.
I popped the hose off the Vacuum reservoir (to the master cylinder) and nothing nasty there. The one-way valve seemed clean and functioning.

I’m thinking the servo isnt working but before I start pulling everything apart, I thought I’d ask you guys what you thought and whether there are any other tests that may determine where the problem lies?

Thanks fellas!

I guess you did check the vacuum lines etc'

Yes - All lines seem OK.
However, is there a way of testing the servo independently from the master cylinder to see if it’s up to scratch?

Does the vacuum tank ever suffer from faults and if so, what are the tell-tales signs?

Cheers, Hilton

Any part can suffer from faults, including the reservoir . Even a rust pinhole will create a vacuum leak. Borrow or buy a vacuum gauge and check that the system holds vacuum . If after stopping the engine, the vacuum slowly drops ( or doesn't exist at all ) then you have a leak. The most likely cause of this ( apart from obvious causes such as a ruptured vacuum pipe ) is failure of the servo diaphragm . To check for this , start the engine and then stop it , and get someone else to press the pedal repeatedly while you bend down near the servo - you should be able to hear a knock from the servo with each release, ( and feel it with a finger lightly pressed on the large daimeter of the servo chamber ) diminishing in volume as the vacuum is exhausted. No knock = no vacuum. It's not a huge job to remove the servo and strip it although it does mean bleeding the whole system again

Thanks Christopher.

Very useful advice
I’ll try all this out and see where I get.

Cheers

Well, I finally got to the bottom of it!

Going through every hose, line and connection, I’ve discovered that the clowns who did some refurbishment work under the bonnet a while ago evidently put the Vacuum Valve back in the wrong direction. Why/how would you do that??? Another item on the list of their botches.
Anyway, a switch around of the valve direction and a quick spin round the block reassured me that actually, the brakes on these Jags are pretty damn good!! All back to full power. Serious stopping power!

Anyway: A happy (and very cheap) ending.
Thanks to everyone for your replies and help.

Cheers, Hilton