Spitfire Bleeding Rear Brakes - Ran Into Issues

[Firm]

It seems every time I go to bleed a braking system, I run into trouble. Here's the details:
I am trying to get a 79 Spitfire back on the road after sitting for something like 5-10 years. When I got the car there was no pedal pressure, so I pulled the m/c and rebuilt it over the winter. Today I:
*Reinstalled the m/c and started bleeding at the right rear wheel cylinder - got nothing but a few dribbles of fluid.
*Moved to the right front caliper and bled - bled easily, getting clear air-free fluid
*Moved to the left front caliper and bled - Again, bleeding perfectly
*Tried the left rear wheel cylinder - nothing, not even a dribble.
Now, my first thought was that the wheel cylinders where seized and not letting the fluid flow though, so I've pulled both of them and will rebuild over the next week. However, I got curious I and tried pushing the pedal with both rear wheel cylinders removed just to see if I could get some fluid to flow out of the lines. Well, I was pretty surprised to see nothing! So I did a quick inspection of the lines and didn't find any kinks or crimps.

So now I got thinking...When I ran into this same issue on my MGB last summer, it turned out to the a pressure differential switch that had been tripped, except on the MGB the switch is located right on the m/c itself. So my question is; does the Spitfire have a pressure differential switch on the braking system? If so, where is it? And most importantly, how can I reset (or bypass) it?

Any help is greatly appreciated, this is the only thing keeping me from being able to take the car out for a test run!

 

[dklawson]

I am not sure about a '79 but earlier Spitfires from the 1970s did have a PDWA. They are mounted horizontally on the bulkhead and located near the coil and heater control valve bracket. I will post one image of the PDWA below (not mine... just borrowed from the net). Google for "Spitfire PDWA" and select images to see more. Also, follow the brake lines down from the MC and you will reach the PDWA quickly.

The wheel cylinders being stuck would not have caused bleeding issues. The problem may be with the MC you rebuilt. The cylinder has a 2-step bore and you may find that one portion is not sealing or that the tipping valve is not working correctly. Bench bleeding may help clear any trapped air pockets if the seals are installed correctly.

 

[RJS]

Not that I know Spitfire brake systems but, I would suspect flexible brake hoses are swollen on the inside. Hopefully that is all. Should be a cheap and quick replacement to remedy if so.

Bob

 

[Firm]

Thanks guys, I did a little reading last night and it sounds like some people report the PDWA actually cutting off the one side of the brake system that is low pressure. There's a thread on Triumpexp where it's debated whether or not the switch can actually cut-off the fluid, but I think it my case it is. I think this may be what happened here.
So, what's the best way to reset, or better yet bypass this thing?

I am really hoping it's not my master cylinder rebuild - It's not the first time I've done this, probably my 5th or 6th time by now and I am always very meticulous about making sure everything goes back in correctly, seal cups the right way, etc.

I need to get this thing moving though, it's blocking some other vehicles that I need access to, so I'd like to get the brakes working asap so I can at least move it around.

 

[dklawson]

I totally overlooked the possibility of the flexible brake hoses. By all means, disconnect the flex lines from the metal pipes and try pumping fluid again. As Bob said, the hoses tend to swell closed on the inside over the years.

The PDWA does nothing that can/will shut off the supply of fluid to a wheel cylinder or caliper. You have already found some references. At one time the Buckeye Triumph web site had a couple of good articles on the function and rebuilding of the PDWA. Succinctly, there is a double ended piston inside the bore of the PDWA. If somehow your car developed an opening on a flex line or wheel cylinder where the fluid did not build pressure on one side when the pedal is pressed, the double ended piston moves to one side or the other inside the PDWA due to the pressure imbalance. The piston actuates the mid-mounted switch causing it to "close". That in turn allows the brake warning light to turn on.

Again, follow Bob's advice first and check that the rear flex lines are open. They should be replaced anyway if they are old and they can cause the problem you are seeing.

 

[Firm]

I've run into that with the flex lines before too on other cars, so it's very possible. Last time though I was able to see the lines bulge, and I was able to get some (although limited) fluid out of them.
I am not sure why I didn't think of this yesterday when I was working on the car; but I think the best way for me to approach this is just to start at the m/c and systematically check for pressure all the way to the back. I'll disconnect the main line at the master and if the m/c is good, it should shoot fluid out right there when the pedal is pushed. If it does I'll move to the PDWA and check for fluid coming out of that. If it's good there I'll move back to the flex lines and check for fluid before those...Pain in the butt, but it's probably the most certain way to pinpoint where my issue is.

Thanks for the help gents

 

[Rhodyspit75]

The master cylinder has two sections, in order for the rear brakes to work you have to fill the rear section too . If you fill the front to the top and keep adding the fluid will flow over into the rear. Or you can pour directly into the rear section.

 

[Firm]

As an update; I think I've solved this issue.

I tried messing around with the pressure differential switch with no luck...Rear 1/2 of the system was still bone dry. So I disconnected the rear feed line directly at the m/c and had the wifey pump the pedal while I held my finger over the port. No fluid flow at all from the m/c out of the port to the rear of the car

(which happens to be the port closest to the front of the car)

I had rebuilt the m/c myself and I am always incredibly meticulous about making sure it all goes back together correctly. Fortunately, i still had the detailed pictures I took of the disassembly. So I took the whole thing out of the car once again, opened it up and triple checked that all the internals had been assembled correctly, everything checked out 100%.

I reassembled it and loaded the tandem internal pistons and proceeded to bench test it....seemed to be working so I re-installed in the car. Almost immediately I began to get fluid flow to all corners of the car. Thank God because I was just about starting to lose my mind.

I am still not sure what the issue was, but I suspect it had something to do with the little pin that keeps the rear piston loaded in the cylinder...Maybe I didn't push the piston back far enough before installing the pin when I initially re-assembled the unit?

Now that there's fluid freely flowing to all corners it should be a piece of cake to bleed the system once I re-install the rear wheel cylinders I finished rebuilding yesterday.