Brake problem mk2 1968

Thanks all for your reply on my question regarding sometimes brakes on during driving.
As I said I renewed all brake hoses to the brake cylinders, also renewed the brake master cylinder. It is strange that as it happens and I pressed the brake pedal just a little the brakes are off. The connecting lever from the brake pedal to the master brake cylinder is still the original.As I put my finger on the outlet of the air valve with is connected to the servo unit and by running engine it sucks so air is flowing into the air valve and to the chamber of the servo unit.
I still have not the problem solved.
Best Regards, and thanks for all your advices.

Just go for a drive with the vacuum line to the servo disconnected. You will of course have to push harder! But it will show whether the servo is involved.

Since you replaced the master Cylinder I expect this to be the issue.
When replacing the original master cylinder with a aftermarket one you need to install a spacer between master cylinder and pedal housing, What is the part number of the new Master Cylinder?

Hi Peter Jan,

Thanks for the info, The part number of the brake cylinder at Barrat is C27160.
Its a little slumpy of Barrat that they not mentioned it at the sale.
I just was on the road with the jag and the only thing i experienced was that by several times braking at short intervals the brake pedal stroke
shortened, however the brakes worked fine.


On my S-type I had to add this spacer, as the fork was to tight.
Did you check if the brake fork has some space, it should not be to tight.

Hi, I have the same spacer flange fitted under the master brake cylinder.
The connecting pin between the fork and the brake pedal is easy to connect.So there is no tension needed to fit the connecting pin.

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Did you repair or replace the booster?

If the reaction valve piston sticks it will cause what you describe.

That’s not an issue, the master cylinder is most likely.
There should be a bit of slack when you push the pedal, it should not move the pistons in the master right away; there must be a gap between the rod and the piston so the brakes are never ‘slightly applied’.
I never understand what the reaction valve is good for but after a rebuild the rebuilt items are always questionable and it seems that something is sticking. Try the master cylinder on the bench.

The reaction valve acts to let air into the atmospheric side of the booster when you put your foot on the brake. It is actuated by a small subsidiary piston in the master cylinder.

This creates a pressure differential between the atmospheric and vacuum sides of the booster chamber (these are separated by a big rubber diaphragm).

This pushes the diaphragm forwards, and it is attached to a rod that pushes on the hydraulic piston in the cylinder, adding force to the hydraulic pressure from your foot.

Getting back to the problem, the subsidiary piston can get sticky or jam which leaves the reaction valve open and causes the brakes to stay on.

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Oh. I was thinking of the tipping valve in the master. That’s hydraulic and actuated by one of the pistons inside. Modern cars don’t have such a valve but I assumed a 68 Mk2 wouldn’t be so different from a 69 XJ as far as how they did things back then.
But thank you for the explanation, that is another possibility and it would be less pronounced without the booster hooked up, when there’s a much smaller pressure difference?

I would still suspect the new parts first.

Again thanks for all info, I checked the clearance of the brake pedal and it has a free stroke of about 20 mm before I felt resistance, so the the master brake is totally free during driving.
Does anyone has a instruction sheet or manual how the brake system is working as in the Haynesbook it is not mentioned. They mentioned only the take apart and reassembling modes.