Late IIA brakes - fronts locking on

[brookvale]

A simple question for the experts...
Front brake shoes are locking ON.
Actual scenario:
1972 IIA - with headlights in guards; regular IIA 'hard' :o dash; fitted with dual master cylinder and servo but no splitter-with-failure-warning-light gizmo fitted.
Front brakes fed from rear of master cylinder (rear as in next to the servo), the line with the 1/2" union nut on, and rears from front end (7/16" union nut).

Tests:
Front drums are in place; One rear drum is off. Gentle pedal pressure sees the rear shoes move out. Let pedal release; with hand pressure alone, you can push the shoes back; wheel cylinder pistons move inwards; front reservoir brake fluid level rises.

Replace rear drum. Move to the front, remove a drum.
Gentle pedal pressure sees the front shoes move out. Let pedal release; with neither hand pressure or greater, you CANNOT push the shoes back; wheel cylinder pistons stay put; front reservoir brake fluid level stays the same.

Currently this means that the first time you hit the brakes, the front shoes 'lock' on and the pedal stays hard. Undriveable! Both front shoe sets are at minimum snail adjustment; springs in correct places etc etc.

Bleeding either fronts or rears works fine, fluid gets pumped out, new fluid sucked in from reservoir.

Rear of master cylinder has the "tipping valve" (per Haynes description) between reservoir and master cylinder; front end just has open hole.

I have seen one reference web-wise that this master cylinder setup is designed to maintain some 'residual pressure' on the front to keep the shoes 'close' to the drums. The current situation results more than 'close' :mad:

Before I dismantle the whole master cylinder, the Question:
Should fluid from front wheel cylinders be able to freely flow back into the reservoir after pedal is released?

Your expert guidance will be much appreciated
Neil
Auckland

[ian4002000]

It sound like a master cylinder problem to me. Are you sure it is the correct master cylinder ? Is it a new one or has it been on the car for a while ?

[cookey]

Check that you have clearance between the rear m/cyl piston and the booster output rod. (An easy way to check this is to slacken the m/cyl bolts a few turns and see if the problem is rectified). Correct clearance here is critical.

Cookey

[brookvale]

From Series Land Rover Dual Power brake conversion (http://www.expeditionlandrover.info/dual_brake_conv.htm) - the master cylinder matches the 88" version exactly.

I haven't yet dismantled anything so will bear in mind the rod/piston clearance - thanks very much.
If anyone does try the ''squeeze back' process on their front shoes - let me know :D

[Blknight.aus]

for starters try pushing them back with the brake bleeder open.

That identifies mechanical failure of the cylinders VS a hydraulic problem.

Im tipping hydraulic.

repeat the test and crack off the joint at the top of the flex line where it joins the chassis pipe.

keep working backwards untill you reach the four way joiner down on the chassis.

when you open a joint and cant push the shoes back the blockage lies within the section between the last place that let the shoes retract and where you cant get them to retract.

[brookvale]

Thanks - opening the bleed nipple allows the pistons to retract. Bleeding the normal way works fine (from reservoir to bleed nipple) but I hadn't thought of there being a 'one way only' returning blockage in the piping but it's odd that such a blockage would be so complete. I'll check it out though to cover all the possiblities. cheers :)

[1950landy]

You may have a faulty return valve in the master cylinder .

[jakeslouw]

You may have a faulty return valve in the master cylinder .

X 2 on this

[incisor]

check the hoses

they collapse internally and the symptoms are exsctly whst you describe

[Mick_Marsh]

check the hoses

they collapse internally and the symptoms are exsctly whst you describe

I hadn't thought of there being a 'one way only' returning blockage in the piping but it's odd that such a blockage would be so complete.
Very common problem when brakes are locking on. It's usually caused by the flexible brake hoses de-laminating on the inside. It has the effect of a one way valve in the brake hose. I've had that problem on all sorts of cars. If you find that is the case, I would advise to replace all the brake hoses as the others would not be too far off de-laminating.