1970 Corvette 350 cu. in. brakes

I have been messing about with these brakes for months on and off. I postede in May time about the difficulty of getting fluid through to the back calipers. See here:
http://groups.google.co.uk/group/alt.autos.corvette/browse_thread/thread/db4b543c44375b60/a73c8023a0e83272?hl=en&lnk=gst&q=georgewbell#a73c8023a0e83272
I have spent over 1000 dollars on parts so far and I still have a problem.
Four new calipers, the complete set of bits to do the handbrake, new pipes, new pads, lots of odds and ends and so much time that my head spins just thinking about it.
I thought it must be the pressure differential sensing valve, but I took it out and pushed the spool back and forth against the springs and the contact made at the appropriate time AND air could get through both ways, i.e. front and rear when it was at rest. I tried bleeding the front brakes with a vacuum pipe on the caliper bleeding nipples. I made up a special bleed nipple that did not seal but let fluid through all the time. With a big reservoir above the master cylinder and a glass jug between the nipple and my vacuum pump, everything looked fine and I got lots of clear fluid going through. This should have not actuated the pressure valve at all.
However when I tried the same on the rear calipers, it was as if the valve had sealed again, as no fluid could be sucked through. It was too late to do any more, today, but what do you think about that? As far as I know there is nothing else that could stop the flow. Or could it even be the set up of the master cylinder clearances. It WAS working and when I took it all apart it looked in perfect condition so I put it all back as it was. Cleaned out with new DOT 4 fluid, of course.
I am glad to say the lights and wipers and associated vacuum circuits are working perfectly now too. I have not really got that much more to do.
This car has got to go out of my workshop very soon as I want to build a bio-diesel processor exactly where it is now. I has been jacked up for months.
Anyone got any ideas? Please. Regards George.
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http://groups.google.co.uk/group/alt.autos.corvette/browse_thread/thread/db4b543c44375b60/a73c8023a0e83272?hl=en&lnk=gst&q=georgewbell#a73c8023a0e83272
Is it possible you have the pistons in the back brake calipers bottomed out? Did you put on new hoses?
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Hi Dad, I'm not sure what bottomed out means. The calipers were brand new and the same style etc. as the ones that came out. I did not change any of the pipes as yet as they are not bad and I want to see it work before I change anything else. All the oil that came out of various places was old, but clean, so I don't expect any blockages. Even if the pistons are at the bottom of the bores, the fluid would still get in. No, the more I think about it there is something very basic stopping the fluid going through and my reasoning is at fault. I shall take the pipes off the master cylinder and the pressure differential again and see where the actual blocking is taking place. I may take them both to the agent and buy whatever he says is faulty. The annoying thing is that they all seem OK but the front works perfectly and I can't get a drop through to the back end.
I may be too close to the problem and can't see the wood for all the trees!
Regards George.
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.
See? Do not assume anything and start from the simple facts. I assumed that because I read about the pressure differential valve could block off a failing front or back system. So that must be it - isn't it? NO Twit! Another reason that no fluid is getting to either rear caliper just could be two pipes blocked! And so it was.
The flexible pipes at the rear were both solid and I could not even hammer a thin brazing rod through. When I replaced these everything suddenly went right. With my suction pump, I did not even have to prime the master cylinder. The fluid went through so fast I shall have to put a tap on it to regulate the vacuum. I am really looking forward to doing it tomorrow now.
Thanks for listening. Nothing like talking to yourself to get an application of logic.
Regards George.
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Snip
Rememeber me asking if you had changed the hoses? Now you know why I ask.

See? Do not assume anything and start from the simple facts. I assumed that because I read about the pressure differential valve could block off a failing front or back system. So that must be it - isn't it? NO Twit! Another reason that no fluid is getting to either rear caliper just could be two pipes blocked! And so it was.
The flexible pipes at the rear were both solid and I could not even hammer a thin brazing rod through. When I replaced these everything suddenly went right. With my suction pump, I did not even have to prime the master cylinder. The fluid went through so fast I shall have to put a tap on it to regulate the vacuum. I am really looking forward to doing it tomorrow now.
Thanks for listening. Nothing like talking to yourself to get an application of logic.
Regards George.
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Amazing that both hoses were blocked. You would expect only one to be blocked and if you have the same problem on both rears, then the master cylinder or 'proportioning' valve are the only common elements.
Glad you got it fixed.

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