Morris Minor Owners Club

....Do disc brake kits require a header tank? I've heard that the callipers displace more fluid but how can this be the reason? It's your foot that displaces the fluid from the MC and you can't pump more than you could in one stroke of the pedal.

I thought it was that the fluid level can drop more as the pads wear so you'd need to top up otherwise. I've got discs and no header tank and topping up the fluid every year or so isn't a great hardship. I guess it'll overflow when I push the callipers back when I fit new pads but it's silicone fluid so isn't going to harm the chassis, otherwise one could always open the bleed nipple.

Yes - it's just because the calipers are relatively large and as the pads wear, they move outwards and fill up with fluid. You don't 'need' a remote reservoir - I have a sneaky suspicion it's just a sales pitch ! But for those who don't want to (or can't) get down on their knees to lift carpet and inspect level in the master cylinder - I suppose it makes level check easy. Unless it leaks of course - which I gather quite a few do !! Kind of defeats the purpose.

If you're tight like me and buy the cheap kit (the one marketed by a property developer not the one designed by an engineer ) then it's flexible pipe that is the problem. The fluid just leaches through.

In case you didn't get it so far. The pistons in standard brake cylinders return back (that's what the return springs do) and you manually adjust for wear. The pistons in disk barke calipers don't return back so they naturally account for wear. As the pads wear the piston moves more and more and the m/c will run out of fluid.
On a modern car, the reservoir is big enough (between min and max) that you can push the pistons back by hand when fitting new pads without overflowing the reservoir.

Unless it leaks of course - which I gather quite a few do !! Kind of defeats the purpose.

Yes - a lot of remote reservoir kits have been sold with a pipe that is not fit for purpose. You need a pipe that is comaptible with brake fluid (e.g. EPDM 'rubber' material)