Brakes coming on when engine started

[phoenix]

I have changed the valve at the end of the master cylider[newish) had no problem until I replied head gasket. L can only think I have disturbed something. If I remove vacuum from inlet manifold brakes come off. S1 4.2 1964

[mgcjag]

If the brakes come on when you start the engine but don't press the brake pedal then there is probably a fault in the reaction valve...the unit at the end of the master cylinder.....there are great drawings of brake operation and detailed explanation of how the vacume operates the brakes in the service manual brake section.....Steve

[MarekH]

The brake booster gives you vacuum assist when you have atmospheric pressure at the back of the booster and vacuum at the front of the booster.
The brake booster gives no assist when both front and back of the booster see the same pressure.

kind regards
Marek

[phoenix]

Sorted now thanks guys.

[mgcjag]

You could st least tell us what the problem was....Steve

[DWW]

That is good, tell us how to help future such occurrences or others who may experience similar issues.