Brake pedal "easing" to the floor?

[straker]

Hello, hoping for ideas from the hive mind as to where my problem might be

Had a sticky front brake on my MX5 mk1 . . . decided to fit a rebuild kit to a pair of original calipers
to replace the sticky ones, so far so good. After bleeding the brakes and testing the pedal hardness the
pedal would occassionally go to the floor, it still had some resisitance but just kept going, not every time,
maybe once every 8 or 9 pumps . . .
Start the car up so now it has the benefit of the servo, seems fine . . . go for a drive and the problem is back, every five or six
braking moments the pedal would go right down, with me having to pump the brakes to stop! . . . not ideal.
Since I just changed the calipers I thought it must be the master cylinder perhaps re circulating fluid internally somehow?
There are no leaks, all bled nicely, but occasionally the pedal goes to the floor . . . .
Ordered a rebuild kit for the master cylinder as new ones are hard to get and take time, fitted the kit easy enough, nothing
obvious wrong with the old seals but in went the new ones and all back together and bled.
Now the pedal is hard for a moment, and then sinks steadily to the floor every time . . No leaks I can see,
bled 10 times to make sure, but still goes to the floor . . . . . . I swapped the old calipers back and re-bled, still the same.

I thought if it were just air then the pedal would pump up and hold pressure? Again no fluid is leaking anywhere yet the pedal goes down . . . . all the flexis are stainless steel braided hose too . . . . . all 4 wheels "lock" when the pedal is applied . . .

I'm resigned the throwing parts at it, perhaps another master ( used ) . . .

I'm hoping for a eureka moment from somebody here, Ive been staring at it too long! . . . .

thanks . . .

[LennyM1984]

Does the MK1 MX5 have abs? Air can get stuck in the ABS pump and it can be hard to extricate it. On my more modern cars you can activate the ABS pump via OBD to bleed it

[straker]

Its not got abs . . . .

[E-bmw]

Servo leak, check the vacuum pipework to the servo, if that is OK, the servo diaphragm is likely holed or the casing rusty & porous.

I am making the assumption that the engine is running at the time obviously.

[straker]

thanks for the replies so far . . .

the issue is there with and without the engine running . . .

[E-bmw]

straker said:

thanks for the replies so far . . .

the issue is there with and without the engine running . . .

OK, that shows the servo basically isn't working.

[simon_harris]

I had an issue with a clutch master cylinder in a Rover SD1 that was similar issues - ended up being a failing rubber on the end of he rod inside the master cylinder.

I assume you have checked that you are not losing fluid anywhere?