discs etc

[Peetee]

I've just fitted a new M/C and Ford discs to Joe. There is a lot of travel on the pedal and I don't understand why. I would expect it to be spongy (which it is a bit) but not the excess movement. What's worrying me is that it feels the same as it did on the old M/C and drums - which is why i gave up and bit the bullet. I did find in the subsequent bleeding (which I have done 3 times now all round) that one of the rear unions was loose, but that's tightened up now. As yet I have to finish fitting the top up kit. The pipe is in place, routed into the engine bay and capped off (the bottle had fractured and has been sent back to the supplier).

Is it just that I need that head of pressure in the top up kit to make the disc calipers take up the slack? Its just we are talking almost the whole pedal travel!

[rayofleamington]

Is it just that I need that head of pressure in the top up kit to make the disc calipers take up the slack? Its just we are talking almost the whole pedal travel!

The head of pressure should be of negligible effect.
The additional reservoir is there because your m/c could be empty if you you let the pads and disks completely wear out and didn't check the fluid level.

where are the bleed screws located on the calipers? If they are not right at the top then you stand a chance of having trapped air regardless of how much bleeding you do
This can get gradually mixed into the fluid so repeated bleeding after you have done many miles can help.

[bmcecosse]

It's a common Mini mistake to put the calipers on the wrong side - with the bleed screws at the base. As Ray says - they need to be at the top. Have you taken the little washer out of the master cylinder ok ?
The joys of discs!! Thank goodness for nice big trouble free drums!!

[bigginger]

Good Lord! I never knew you thought that...

[Peetee]

As Ray says - they need to be at the top. Have you taken the little washer out of the master cylinder ok ?

Yes and yes.

[jonathon]

I presume that you are not loosimg pedal pressure,and just have a soft pedal. Where is the biting point. You can adjust the pedal rod to bring the pedal higher if need be. I have always found non servo'd discs to feel spongy, but thats probably because I'm used to servo's.
You could try putting a brake pipe clamp on the rear flexi hose and see if this improves the pedal, if it does then the problem is clearly at the back, if not then move to n/s/f then o/s/f flexis.
As Ray says the header tank will make no difference in what you are experiencing, unless its sucking in air,or there is no flyid at all in the supply pipe. If you have blanked off the pipe to the header then you will have removed the venting which the header requires, so maybe causing a vacuum ?, whether this is enough to give you a problem I'm not sure. Try bleeding again once you have a complete system.

[Alec]

Hello PT,

are the calipers new and\or reconditioned or second hand? If the latter, is it possible you have a seized piston in one caliper which could give a longer pedal plus the real danger of fracturing the disc?

Alec

[Peetee]

They are supposed to be new along with everything else, but the box is marked reconditioned

[Willie]

Peetee, I have just fitted Jonathons' disc brake kit and I too was surprised by
the increased pedal movement and difficulty in bleeding the system. I bled
the system three times with the aid of my wife without 100% success but,
after a fifty mile drive I bled the system on my own by undoing each bleed
nipple until brake fluid started to seep out and then closing them up. The result was a much better feel, not as hard as the drum set up but the brakes
are excellent. There is more movement of the pedal before the brakes bite
probably due to the need to displace more fluid for discs, this is one area where the early master cylinders with their larger 7/8" bores would probably
be more suitable for use with disc set-ups. My system is servoed and I find
the conversion well worth the effort. A word of warning re' adjusting the brake pedal rod', It is absolutely vital that there is a very small amount of
free play before the rod starts to move the master cylinder piston. If you find
that you have quite a bit of free play then you will probably find that the clutch pedal shaft and the brake pedal bushes are badly worn. The operating
rod does NOT have a lot of spare thread for adjustment before the rod will
unscrew right out of its housing which could be fatal.

[Peetee]

Cheers mate I'll give it a bleed again tonight. I had the same set up before on a previous car and the travel was fine - but it was a 948 with the different M/C as you have described.