Brake question

[Chrisrich]

Hello Forum.
Trying to reconcile the brakes on my '70 S4 Elan.
From the beginning (2 years ago?) I noticed sub-par brakes. I'm finally trying to tackle the question of where my brakes have gone. I've turned the rotors, replaced the pads, checked for leaks, bled and re-bled (no bubbles), and yet I've still got very anemic brakes. From 40 or 50 mph, for instance, it is impossible to lock the brakes. I'm turning my attention to the plunger in the tandem cylinder: perhaps it is off-center. The manual describes a method to observe that the plunger: using the brake failure indicator light. I have no such light, it seems, and further, I don't know by what means any light switch would be tripped.

So here's my question:

Is there a straight-forward way to reconcile the position of the plunger, and actually know if it were to go askew WITHOUT the benefit of the warning light?

And secondly, does the fact that I can push brake fluid to all 4 wheels indicate that the plunger IS centered?

Thanks a million-
Chris

[Frank Howard]

I'm turning my attention to the plunger in the tandem cylinder: perhaps it is off-center. The manual describes a method to observe that the plunger: using the brake failure indicator light. I have no such light, it seems, and further, I don't know by what means any light switch would be tripped.

So here's my question:

Is there a straight-forward way to reconcile the position of the plunger, and actually know if it were to go askew WITHOUT the benefit of the warning light?

And secondly, does the fact that I can push brake fluid to all 4 wheels indicate that the plunger IS centered?

Chris,

First of all, I think you've got the terminology mixed up. The tandem cylinder is the brake master cylinder. The one with the plunger you are talking about actuates the brake fail light when you lose pressure in either side of the tandem brake system. It contains a switch that has two wires that go to the brake fail light and to a hot power supply. When you lose brake pressure in half of the system, the pressure in the other half causes the plunger to move which in turn mechanically depresses the switch. To answer your questions:

1. There is a way to reconcile the position of the plunger without the benefit of a warning light.
2. There is a way to know if it were to go askew without he benefit of a warning light.
3. The fact that you can push brake fluid to all 4 wheels does not indicate that the plunger is centered. The brakes will work just fine even if the plunger is pushed to one side.

[stugilmour]

Chris:

There are a couple of pic's of the PDW Valve (Pressure Distribution Warning) on the thread link below if you are having trouble locating it. It is the deal with the plunger in it that Frank is describing above. I could not find a pic or service instructions in the Plus 2 books, but bleed procedure is written up.

elan-f15/master-cylinder-disaster-t19071.html

Frank:

The Lotus Plus 2 Federal wiring diagram seems to show the ground on the PDW going to the body of the valve. Mine has a small plastic connector on the top with two small pins. Not sure if it is a bodge connection done by the PO, but they both run to the warning light using a single large female spade connector around both pins.

From your description above, sounds like mine is electrically correct?

Did it come with a proper plug connection originally or just the spade?

I am assuming there is one pin for each direction of plunger displacement, and circuit is completed to ground on valve body in case of failure of either front or rear hydraulic circuit?

thx

[Chrisrich]

Thanks for that input, folks.

I don't seem to have a PDW valve, although I have a federal car. I have one in my TR6, so I get the idea of it, but once the two pipes leave my master, they go their separate ways.

Meanwhile, I've removed the master and it seems there is plenty of positive pressure at the rear outputs, but almost none at the front, leading me to further suspect the plunger. This master was rebuilt by Whitepost, so I guess I'll give them a call for some support.

Chris