Some advice on strange brake behaviour please.

[lestere]

Having just put the E back together after left to RHD conversion I was surprised by very hard brake peddle and ineffective brakes. The master cylinder was moved and the large bore hydraulic pipe joining this to the servo slave cylinder connected. I have checked and rechecked the rerouted the vacuum lines according to the Haynes manual and parts book. All this appears correct for a later 4.2 system.
The symptoms I have are:
Very hard brake pedal when the engine is running.
Brake peddle is easier to press when the engine is off and vacuum at zero.
I have vacuum at the servo front vac connection and master cylinder.
The vacuum at the servo rear connection reduces when the peddle is pressed very hard.
When I turn the engine off the system holds vacuum.
When I press the brake pedal with the engine off I can hear a hiss at air is admitted at the reaction valve.
When I remove foot from the brake pedal I immediately hear the servo return under spring pressure.
I can do this at least five times with the held vacuum.
When I remove the cap from the reaction valve on the master cylinder and someone presses the brake pedal I can see the valve working as the air at atmospheric pressure is admitted (I use my finger to replicate spring pressure).
If I allow air at atmospheric pressure to enter the brakes are applied very slightly at the front (I can still turn the wheel by hand) and more effectively at the rear.
I hardly drove this car before the conversion. I was aware the brakes were not good before conversion, but they didn’t seem this bad. I keep thinking it’s something may have refitted incorrectly. Possibly they were, or a previous owner error??
Any all advice from the collective will be very gratefully received.
Thanks in advance
Lester

[christopher storey]

The first thing I suggest is that you study carefully the diagrams at figs 1 and 2 of pages L.Y.s.2 and s.3 in the workshop manual I don't know if these are in haynes , but they are available here on the knowledge base . The link is http://redirect.viglink.com/?format=go& ... f%3Fdl%3D1

What is happening sounds very much like the reverse of what should be happening, which is that the pedal should go hard once vacuum is exhausted, but should go soft when vacuum is applied . This is part of the standard MOT test now done , whereby you should feel the pedal "give" when the engine is started with the pedal held down. I have a suspicion that what is happening is that the situation in the two chambers of the servo is reversed ( E and F in the diagrams ) possibly because the pipes from the reaction valve , 1 to the servo rear chamber, and 1 to the manifold and in parallel to the servo front chamber via the reservoir , have been transposed. ( Or possibly because there is a fault in the reaction valve )

I say this because with the pedal pressed , the rear chamber should not show a reduction in vacuum as you are getting, but should be vented to atmosphere ( i.e the vacuum disappears altogether , thus providing the pressure differential which applies extra force to the shuttle in the servo cylinder

[mgcjag]

Hi Lester....do you have the Jaguar Service manual...if not its in the knowledge base section( technical documentation) ...there are some very detailed diagrams of the braking system showing vacume and hydraulic flow.....note that the connections did change and are shown in the diagrams....PM me if you strugle to find them.....im sure with these diagrams you will be able to sort your problem...all the best.... Steve

[lestere]

Hi Christopher and Steve.
Many thanks for the link the workshop manual and advice. Yes, the diagrams are the same as the Haynes manual and as on my car. I did MOT servo test. My pedal does not move further down as expected when pressed and then the engine is started.
I think I have confused you by saying the vacuum to the rear of the servo is reduced when the brake pedal is pressed. I mean there is less vacuum and it moves closer to atmospheric pressure.
When I remove the vacuum pipe to the rear the servo so this goes to atmospheric pressure the brakes are pulled on, very slightly.
I have very carefully rechecked all the hydraulic and vacuum lines. All correct. I will check again tomorrow with fresh eyes.
Thanks again

[christopher storey]

Lester : it is also worth checking the non-return valve for correct fitment/leaks and the vacuum reservoir for leaks, because if the rear chamber is going fully atmospheric when the pedal is pressed, then the other possibility is that the front chamber has not enough vacuum in the first place. Somehow or other the proper differential between atmospheric and vacuum is not being established in your case

Your "moving closer to atmospheric " in relation to the rear chamber concerns me. That just does not sound right, and perhaps points to a reaction valve fault : the rear chamber really should be an all or nothing case with no intermediate stage. Alternatively, I am just wondering whether the rubber diaphragm in the servo itself is leaking/perforated, which would allow vacuum to be exerted to a slight degree in the rear chamber even when the pedal is pressed

[lestere]

Thanks again for the advice, very helpfull.
I’ve been investigating the brakes further. Bled the front brakes again, disconnected all the vacuum lines and checked them for damage and possible collapse. The system is holding good vacuum which is opened to atmospheric pressure by the reaction valve, so non return valve and reaction valve are working. I can feel and hear the servo moving back and forth.
Now its all back together (again) it’s better, and better still after a drive. There is still a firmness to the peddle before it is pressed enough to open the reaction valve. Then the servo comes into play and brakes function more as expected.
Interestingly, when looking back through some of the old invoices that came with the car I see the owner asked a garage in Oregon to check out the brakes in April 91. They report, “bump in master cylinder is internal, must remove master cylinder and inspect, suggest driving and report any problems”.
Looking further back I see the master and servo cylinders were fitted with new seals in August 1980.
So, my next step will be master cylinder and servo repair kits and a rebuild. Hopefully the cylinder bores will be in serviceable condition.
Thinking about this a bit more while typing …….. I will pull out the master and slave cylinders and inspect their condition before buying the kits. Just in case there is a step in the bore. Hopefully its just the little flap valve at the rear of the master cylinder sticking???