Brake pedal going to the floor

[scotty]

Hi guys, this is my first post as I've just recently joined. 

My friend and I are currently building an mnr vortx with mx5 running gear which we bought as a half finished project.

The problem we have is with the brakes, we have the big brake kit from a mk2.5, when we fitted everything and bled the brakes through thoroughly the brake pedal will go to the floor the first press, after that you get a good pedal , we have checked for leaks everywhere but can't find anything, we have adjusted up the Allen bolt at the back of the rear calipers for the handbrake as well and it has us stumped, any ideas guys?

Regards

Scott and Maz

Edited February 26, 2023 by richyb66
Removed empty lines from post.

[richyb66]

Do you have the correct master cylinder for the bigger brakes? Sounds like it isn't moving enough fluid.

[scotty]

No we don't actually , the brake reservoir set up is from a mk2, I didn't realise that would make a difference 

 

After looking online I don't think there is a difference in master cylinders between the mi2 and the mk2.5 so it must be something else

 

 

 

 

 

 

 

 

[RallyChris]

Just a thought - have you retained the original MX5 proportioning valve alongside the master cylinder?
 

My understanding is that acts as a shuttle valve in the event of a loss of pressure (I.e leak) in one of the circuits (front or rear), to shut off the downstream pipe work so it doesn’t pump fluid out in the event of a leak.
 

if you have this valve fitted, I wonder if your issue is that the first pump of the pedal is activating this valve due to air (or leak) in one of the circuits, so your initial pedal movement is lost to push the valve over, and after that you can pump up the ‘good’ circuit to get a hard pedal.

Edited February 26, 2023 by RallyChris

[Sparepart]

Just a thought. Before you press the brake pedal, check the gap between the back of the pad and the piston and the pad and the disk, see if the pads are quite free. Do your pedal pumping thing until the brakes are firm, now the pads should be firmly gripping the disk. Then release the brake pedal. Now there should be a residual pressure in the pipe that keeps the pads very close to the disk, actually very slightly still braking. If the pads/pistons retire away from the disk then this could explain your first ineffectual pump. You might find no real difference in the piston/pad freedom in which case it would rule out a lack of "residual pressure" being your problem.

Edited February 27, 2023 by Sparepart
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