Resetting PDWA after bleeding brakes

[collins_dan]

I'm about the rebuild the master cylinder, replace rubber brakes hoses with stainless steel and bleed the system. My S4 doesn't have a servo, but does have the dual circuit (front and rear) master cylinder with a PDWA. I understand that bleeding the system usually trips the PDWA, how do you recenter the piston? I've heard of opening the rear bleed nipples and lightly tapping the brakes and it clicks back into center. I've heard of removing the PDWA switch and using an awl or small screwdriver to push it back to center. What seems to be the best method? Thanks.

Dan
'70 S4 SE (Federal)

[collins_dan]

After further investigation of the owners manual, it seems that Lotus uses a different brake pressure differentiation warning system than Triumph, but I can find no specific references as to how it works anywhere. Can anyone provide any pictures or explanations as to how it works? Thanks. Dan

[Frank Howard]

Dan,

For all of us who are less knowing, what is a PWDA? What does it stand for? Are you talking about the component that pushes a piston to one side or another in the event of the sudden loss of brake pressure in one or the two brake circuits? If so, are you sure you have this on you car? My '74 Europa has one however my S4 does not and it is slightly newer than your car. It came with a Sovy device that simply lets you know if the brake master is running low. I now how to set the piston on the Europa but before I go into the explanation, please confirm that you have this system rather than the Sovy device. Thanks.

[collins_dan]

Triumph refers to it as a Pressure Differential Warning Actuator (PDWA). In the Lotus manual, its referred to as a Pressure Differential Warning Valve. I assumed (perhaps falsely) that all dual circuit master cylinder cars with brake warning lights had them. In the manual, when it describes bleeding the brakes, it mentions that the warning light is often activated if you are not careful, in which case you need to open the bleeder screws on the opposite part of the system (back if you are bleeding front, front if bleeding back) and tap lightly on brakes to reset the valve, which you will feel click back into place. I'll look when I get home tonight. What does the Sovy device look like in the MC? I don't recall seeing anything that looked like a float in there and there are no wires coming out of it to the dash light. Where is the piston located on your Europa? Thanks. Dan

[patrics]

Hi Dan

The PDWA is a self centering device so should sort it's self out - but might not after this much time.

If it were my car I would remove it and replace with two three ways.

I will scan a how it works and post tomorrow.

Cheers
Steve

[Frank Howard]

Dan,

The Sovy device is screwed into a can that is screwed into the master cylinder reservoir where the cap would normally be. If yours is missing, that is not unusual as very few survive and nobody makes a replacement.

If you are seeking help with your PDWA, it would probably make sense for you to determine whether or not you have one first. That way, if indeed you don't have one, you won't be asking someone to take the time to explain how to fix something that you don't have.

The factory placed the PDWA in the engine compartment right below the servos in my Europa but I have since relocated it to the front of the car near the master cylinder. It is not a self-centering device. If the piston has moved to one side or another, it will stay there even if you regain brake pressure in the side that allowed it to move.

[collins_dan]

Frank,

It would appear, based on the wiring, that at one point the car had both a Sovy and some kind of brake switch, neither of which still exist at this point. There is a loose red/black wire in the engine compartment for the Sovy, and there is a loose red/black wire under the dash, that according to the electrical plan should go to a brake switch, no idea what or where that would be. I also traced the brake lines from the engine compartment (so not that thoroughly), and what I found was the lines run across the frame from the MC across to the right side under the carbs where there are 2 unions, a 3-way for the front and a 2 way for the back. Is that similar to how your's is set up?

Appreciate the help,

Dan

[patrics]

Hi Dan,

As promised please find attached "how it works" for the PDWA

Regards
Steve

[Frank Howard]

Dan,

Now that you have traced the brake pipes for the front and back and have determined that they do not meet, I can safely say that you do not have a PDWA. The reason the pipes go from the master cylinder across the vacuum part of the chassis, and beneath the carbs before heading to the front and rear is because Lotus initially set up your car for right hand steering. Where the unions are is where the pipes were supposed to go into the right hand mounted master cylinder. Once it was determined that the car would have left hand steering, Lotus simply added the unions and extended the brake pipes to the front, across the vacuum part of the chassis and back to the master cylinder now situated on the left side. The problem with this arrangement comes into play when you are bleeding your brakes. Normally, you start with the wheel that is furthest from the master cylinder and work your way to the closest one. On you car, you would assume that the rear right is the furthest however because of the brake line arrangement, the rear left is. You would think that the rear left would be the second furthest however the front left is. You get the idea. Because of this confusion, I re-routed my brake lines years ago. In addition to removing all of the redundancy, I re-routed the pipe to the rear. Mine originally was routed along the bottom of the chassis on the right. Now it resides high up inside the backbone on the left side. The factory even provided grommets for the holes in the bulkheads. I think I removed something like 21 feet of pipe. I no longer have 3 pipes lying on the vacuum part of the chassis, only one. The brake bleeding sequence is much more logical now.

You are correct that the red/black wire is for the Sovy device which most likely vanished long ago . At one point in time, it was connected to the red light on the dash. I know you'll find this hard to believe, but there was a push button switch attached to the black under dash trim that was there to test the brake light warning light. That's the brake switch you are referring to. Look at the bottom of the under dash trim just to the left of the steering column and you will find the hole that the switch resided in.

Hope this helps.

[collins_dan]

Well I'm glad I asked the question, even though what I was concerned about proved to be a non-issue.

So, bleeding the brakes... start with left rear, then left front, then right rear, then right front...correct?

Based on all the bleeding procedures that I'm hearing, how does this sound... loosen up the nipple enough to let out air, slowly pump the brake all the way to the floor and repeat until fluid comes out, then close the nipple before releasing the brake. Go around once using this procedure. Go around a second time in the order described above being very careful to only open the nipple slightly, one steady pump to the floor and closing the nipple before releasing.

Correct...?

Thanks. Dan

[Frank Howard]

So, bleeding the brakes... start with left rear, then left front, then right rear, then right front...correct?

Dan,

You've got it. Until you re-route the brake lines like I did, you will have to figure out which one is the longest (left rear) and work your way to the shortest (right front).

As far as bleeding is concerned, when you loosen up the nipple, brake fluid will drip out. When your partner begins to pump the brakes, a combination of brake fluid and air should come out. When this turns into pure brake fluid, you've removed the air. I'd go aroung once and then test the brakes. If they feel mushy, go around again. I also attach a piece of clear tubing to each of the bleeder screws. This way, it makes less of a mess because you can direct the fluid into a pan. You can also see the bubbles of air as they come out with the fluid. Don't forget to keep topping up the master cylinder. If it empties, you'll have to start all over again. Good luck.