Brake Booster Info

Date: Wed, 21 Oct 1998 06:09:11 -0400
From: Jim Altman
Subject: Re: TR6: Tardy response to Joe Flake's Brake Booster

Made curious by the discussion I looked in Bentley's only to discover no information on the function of the booster. Autopress book on TR5, 250 & 6, 1967-1973, on the other hand enlightens the function as follows:

"Normally the vacuum from the inlet manifold acts on both sides of the diaphragm, but when the brakes are applied the pushrod closes the vacuum port to the rear of the diaphragm and instead opens a port to atmosphere. The differential pressure then moves the diaphragm forward and assists in applying the brakes. The reaction of the master cylinder pressing on the reaction disc forces the center of the disc to push the pushrod back again partially closing the atmosphere port. As the pressure on the reaction disc is dependant on the pressure from the brake pedal the amount of assistance given by the servo will vary with the pressure on the brake pedal.

In the vent of a vacuum failure there is a direct link to the master cylinder through the pushrod, reaction disc and hydraulic pushrod.

Routine maintenance is confined to changing the air filter whenever the rear brake linings or front friction pads are renewed. Pull back the dust cover and remove the old filter from the diaphragm plate neck. Replace it with a new filter and pull the dust cover back into position. To facilitate assembly cut the new filter diagonally from the outer edge to the centre hole. Examine the dust cover and if it is damaged renew it.

If the air valve has failed it may be renewed (thats the connector the hose that the manifold attaches to -ja). Note the angle the old valve makes with the body of the servo unit. Press it down and turn it anticlockwise one third of a turn and lift the valve out. Fit a new O-ring to the valve and fit the assembly back to the servo unit.

The only other part of the servo that can be serviced is the seal and end plate assembly. A service kits is available (still? -ja) containing a new filter, dust cover, nonreturn valve (same thing as the "air valve" above -ja), seal and plate assembly as well as grease for the seal and plate assembly. (The seal and plate assembly is the seal and plate attached to the pushrod that reaches into the master cylinder from the servo - ja). The old seal and plate assembly is removed by gripping the centre rib with a pair of long nosed pliers. Lubricate the new assembly wit the grease provided before fitting it in place.

Place a straightedge across the front shell recess and check, with feeler gauges the gap between the straightedge and the end of the pushrod. If this gaps varies beyond .011 and .016 inch (.28 to .41 mm) then the whole servo must be renewed. Similarly any other faults beyond those mentioned will require the renewal of the complete unit. (Maybe in more idyllic times -ja)"

Thought it was interesting and that I'd share it.

Jim Altman Illigitimi non Carborundum
http://www.altlaw.com/metro/jaltman.html
69-TR6#CC28754L W4UCK