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Spitfire Brake problem 78 Spitfire

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AustinJim

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All,
I'm resurecting a 78 Spitfire that spent 11 years in a barn. I'm almost ready to get it on the road except that I have a brake problem. The brake pedal is soft and goes to the floor until it's been pumped a couple of times. Once it's not going to the floor any more holding pressure on the pedal causes it to slowly move toward the floor, as if it's not holding pressure. Also, once it's been pumped up and is momentarily holding, if I back off the pedal just a little and then press again it goes easily to the floor. As if there is a valve that's causing the pressure to be released when the pedal is backed off slightly.
I'm thinking that the inside of the master cylinder is too pitted and so fluid seeps arount the plunger seals. I recently honed the master cylindar and replaced the rubber parts. I rebuilt the front calipers and replaced the rear wheel cylinders. There are no visible fluid leaks.
What do you think is the problem?
 
So assuming that you've properly bled the system to remove all the air (not easy) then you're right its the master cylinder.
Not familiar with the spit but assume it's similar to most other LBC's, I would guess that the most likely problem is with the valves inside the MC assembled incorrectly or not seating properly, particularly the tipping valve in the top. You could also be right that it's wear in the MC bore but I think that's less likely from your description. You might be better off just getting a new or reconditioned MC, you definitely need brakes!.
 
Since you had the calipers rebulit and replace the rear cylinders go on and replace the master cylinder since you see some pitting. The fluid is leaking past the seals.

Don
 
Who was the vendor and/or manufacturer of whatever "kit" you used to rebuild the master cylinder? Could be there was a bad seal in that kit or a seal got damaged during the rebuild or reassembly.

Also, do you get any braking at all when you first push on the pedal, or do you have to pump just to get something?

Do you see bubbles in the reservoir when "holding" the pedal?
 
My experience for rebuilding the M/C on my spit, is I can do it once but the next time the cylinder is too worn for a second rebuild to last. Then as a sign of what you have, when you bled the lines. Did you have good fluid flow out of the slave cylinders or did you have to pump it up to get a good flow...
 
I went ahead and ordered a rebuilt mc from Apply Hydraulics. They had it in stock and I had it in 2 days. Went on fine and solved the problem. Now on to the hard-to-start issue!
Thanks for the advice!
 
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