Our '72 Spitfire should have a nearly identical brake MC. I did not bench bleed the MC when I installed it (new, not rebuilt). If you are able to get fluid out of the bleed nipples at all 4 corners, the MC would appear to be primed adequately.
The part of your post that caught my attention was the part where you said the pedal would sink to the floor. If the fluid level in the reservoir is not dropping, that sinking pedal suggests fluid getting around the seals in the MC. Is this a rebuilt, NOS, or new MC?
I was annoyed when we bought the expensive MC for the Spitfire because it came with a warning not to use DOT-5 fluid and... it leaked between the plastic reservoir and the metal cylinder housing. I let the vendor know and their reply bothered me even more. They said that they didn't have too many complaints about brake master cylinders... maybe "only 5%". I don't believe I am unique in thinking 5% is too high a complaint level on a safety related component.
Doug L.
--Previous Message--
: I recently replaced my original brake
: master cylinder with a new unit. I did
: not bench bleed the MC - a number of
: people told me any air should bleed out
: by the normal method with enough
: patience. I was able to get a firm
: pedal after several thorough bleedings
: of the system. Driving the car about 30
: miles I felt like the pedal was
: sometimes a tiny bit soft, but on any
: second tap of the pedal it firmed up.
: The next time I tried to drive the car
: the pedal went straight to the floor. I
: bled the system again, but no luck. If
: I push hard on the pedal it has some
: feel, but usually slowly goes to the
: floor. If I push gently it goes
: straight down. Does this all suggest a
: bad MC? There is no fluid leaking from
: the plunger end of the MC. The car does
: have the PDWA. It was replaced about 15
: years ago. Can the PDWA cause issues
: like this? Or should I go back and
: bench bleed the MC? Any advice would be
: gratefully received.
:
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