S3 E-Type

Below I try to describe a common problem on V12 & 4.2 6Cyl E-types and try to provide an explanation of what's going on.

The problem normally manifests itself as front brakes that drag or stick-on after being used. This is often combined with a brake pedal that feels very hard, without the normal 'sponginess'. A quick dab of the brakes or wiggle of the steering wheel can fix the problem until the next brake application. Often the problem only happens when the car is warm, so on a short drive it's OK but on longer runs it starts to happen. It can get worse and affect both front and back brakes, and if you continue to drive with the brakes dragging then it can cause over-heating of the brakes, boiling the brake fluid (= no brakes), melting the grease in front wheel bearings and burning the diff seals on the inboard back brakes.

The normal root-cause for this problem is the 'Reaction Valve' on the end of the master cylinder. This is the white/yellowish plastic device on the end of the m/cyl which is an air valve and is used to operate the servo/booster.

General Brake System Description 
Master Cylinder
The master cylinder is a single-circuit hydraulic cylinder supplied with fluid from the reservoir on the bulkhead. At the end of the m/cyl is a small piston which moves in reaction to an increase in the fluid pressure (when you press the brake pedal). This small piston mechanically operates the air valve inside the reaction valve & this air is fed to the servo. In the event there's no fluid in this m/cyl (due to some catastrophic failure), the actuating piston that's connected to the foot-pedal will move all the way to the end of the m/cyl and mechanically push on the little piston - this is a fail-safe feature allowing the servo to operate in an emergency mode even if there's no fluid pressure in the master single circuit.

Brake Servo or Vacuum Booster
The servo 'receives' the fluid from the single-circuit m/cyl but also has a second hydraulic piston  (& reservoir mounted low by left F/wheel) to create a dual circuit braking system. Both these hydraulic circuits are within the same fluid cylinder and care must be taken with seal orientation on the servo pistons, recognising this dual circuit function.  So, the servo will distribute the hydraulic pressure from the master cylinder to the 2 braking circuits and/or use air-pressure to boost the hydraulic pressure in the 2 circuits.

The air pressure part of the servo works by having an actuating rod connected to a large 'piston' (actually a rubber diaphragm) which separates 2 chambers. Under normal operating conditions both chambers are at low pressure by being connected via the vacuum reservoir to engine vacuum. When the reaction valve is actuated, one of these chambers is connected to the outside air creating a ~15psi differential/force on the large piston and this force is applied to the hydraulic circuit via the piston rod.

Reaction Valve
This valve controls the air pressure in the rear chamber of the brake servo. Under normal conditions it routes a vacuum from the vacuum reservoir to the chamber. When the brake pedal is pressed the small piston in the end of the m/cyl closes the vacuum connection in the reaction valve & opens a connection to air.

Dragging / Sticking Brakes

Very simply, the air valve in the reaction valve does not seal properly. That means that, even with no foot-pressure on the brake pedal, air is being supplied to the rear chamber of the servo and the servo therefore operates and pressurises the brake fluid in both circuits, applying the brakes either partially or in some worst cases brakes are full-on & car won't move!!