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John Kuzman
10-19-2017, 08:04 PM
Took the 1972 Spitfire MK IV for a long fall drive two weeks ago, and when I went to back in to the garage, the brakes acted as though they were locked up. I immediately jacked up all four corners while in the driveway, and the right rear was locked. The other three wheels spun fine with the expected slight drag.

Had a chance to open up the right rear, and it appeared as though the wheel cylinder was weeping fluid. I decided to change both rear wheel cylinders, shoes, drums and all four rubber hoses. The hoses were at least 13 years old.

Finished the brake job today and went to bleed the system. I adjusted the rear shoes tight against the drums and installed Speed Bleeders to make this a one person job. Started at the right rear, absolutely no fluid flow after countless pedal strokes. Tried the left rear, same result. Went back to the supplied bleed screws and had the neighbor assist in the pump and hold routine. No fluid. Re-tightened all connections, no fluid. The PDWA was eliminated by the previous owner over 13 years ago.

Went to the front and both sides bled fine. What am I missing on the rear brakes? Thanks.

dklawson
10-19-2017, 10:24 PM
How did the PO connect the brake lines when they remove the PDWA?

Did you try loosening the brake line fittings at the master cylinder and pushing down slowly on the pedal until you saw fluid come out around the tube and threads?

Andrew Mace
10-20-2017, 12:45 PM
Forgive what I'm about to suggest, but many of us have unknowingly done this. Are you sure that the rear part of the master cylinder reservoir has fluid? There are two chambers in that reservoir. The front one is rather large, but the rear one is quite small in terms of volume, and it's easy to miss refilling it due to the small opening for it.

dklawson
10-20-2017, 03:11 PM
I forgot about the two chambers! Good point Andy!

Rhodyspit75
10-20-2017, 08:52 PM
I thought the rear chamber was for the clutch?

John Kuzman
10-20-2017, 09:04 PM
Andy and Doug -

Thank you for the replies. I had only a short period of free time to further dig in, but both of your inputs are pointing me in the right direction.

The PO actually seems to have done a quality job of by-passing the PDWA. Looks like he made (or had made) brake union fittings with the proper length pieces of line to simply tie the front and rear brake lines in place of the PDWA.

I have enjoyed British cars for several years, however I do not think I have ever seen a stranger set up than the brake master cylinder reservoir on this Spitfire. The rear brake fluid reservoir is hidden from view unless you know to look for it in an undefined space. The fluid in that small, confined space is pitch black. How do you fill, check, monitor that portion of the reservoir? There is fluid in that back portion, but it does not look clean.

I was able to open the line coming from the master cylinder at the rear of the car at the three way union, and there was a very slight dribble of old, black fluid, then nothing. Had the wife gently press the brake pedal...no fluid. Just a slight hiss. I am starting to think that the rear brakes have only marginally worked for years and my recent rear brake lock up was my warning sign.

Game plan when time permits is to open the line at the rear fitting of the master cylinder and see what happens with slight pedal pressure. I am betting on Doug's thoughts that the rear brake circuit is not moving fluid.

dklawson
10-20-2017, 11:28 PM
Emptying the "back" portion of the master cylinder reservoir is easiest done by removing the brake pipe from the rear of the MC and pumping the fluid out.

My concern is that the PO didn't understand how the PDWA works when they made their bypass. Succinctly, your car has a dual line system where the master cylinder has one circuit for the front brakes and a second circuit for the rears. The PDWA is looking for a pressure difference across the dual circuit brake system. To bypass the PDWA you need one line coupler for the front brake pipes and one for the rear brake pipes.

I would start by filling the rear portion of the brake MC and disconnecting the rear brake line going into the MC. Put rags under the open port of the MC and gently push the pedal down. You should get fluid out. Pump until all the dirty fluid is out of the reservoir and refill with clean fluid.

Then reconnect the pipe and move to your PDWA substitute. You should take it off completely and look at how it is working... and possibly replace it with 2 couplers. Once you are sure fluid can pass through the pipes, move down the line from the MC to the rear wheel cylinders. Open each joint you find and pump the brake pedal slowly until you see fluid come out of the open joint. Close that joint and move down to the next joint. Repeat the process until you've been able to get fluid all the way to the back of the car.

Andrew Mace
10-21-2017, 07:49 PM
I think Doug's covered things pretty well here. I would add that the "presence" of old, icky, black fluid tells me that at least the rear half of the brake system might not have seen much attention since the Reagan administration! You seem to have dealt with almost all of that, but I do wonder still if your master cylinder might need attention?

John Kuzman
10-21-2017, 08:29 PM
Well, the master cylinder, at least the portion for the rear brakes, is shot. Removed the MC today and put it in a vise. Pushed in the pushrod, the front port squirts fluid. Nothing out the rear port. put an awl in the rear port and it came out with a black, tar-like substance. As Andy suggested, the rear system may have had original, Nixon era fluid!

My neighbor is a retired mechanical engineer who, I just learned yesterday, spent his entire career in the design and manufacturing of industrial hydraulic valves and cylinders. He took the MC and is going to take it to his former employer's facility where he will have it disassembled, soaked in an industrial solvent, inspected and rebuilt. Thought he would have it back to me by next Thursday.

To close the part about the by-passed PDWA, yes Doug, the PO used brake unions on both the front and rear lines to keep the two circuits separate.

Thank you to Doug, Andy and everyone else, including my newly introduced neighbor.

Rhodyspit75
10-21-2017, 08:45 PM
I thought the rear chamber was for the clutch?
Whoops my age is showing.:sorry: I know that’s not the case. Presently restoring a Bugeye where one master cylinder serves the brakes and the clutch.