If you press the pedal slowly and bubbles come up then you have a leaking secondary seal - fix this ASAP. Corrosion in the bore making the secondary seal leak is the most common failure of the Moggy master cylinder (it is more likely to happen in cold weather as the seals shrink just a fraction but it can happen at any time). If left unchecked you are likely to get a nasty surprise and no brakes one day.
The open end of the master cylinder bore suffers from damp and condensation and also can get water trapped in it as the washer behind the circlip does no allow water to get out
(therefore don't be hasty when it comes to driving in a flood - if the chassis leg gets a lot of water in it, so will the m/c!). Water in the m/c will make the bore rust and when the secondary starts leaking you can't ignore it.
To get a very good pedal you will need the shoes adjusted to remove
all clearance. If that is already done and the pedal travel is still wrong then there are many other reasons:
1) air in the system due to bleeding
2) wrongly fitted wheel cylinder cup seal (I've seen one upside down! it worked but gave spongy brakes). This is only possible with the cup seal and an upside down piston lip seal shouldn't give a springy pedal.
3) air in the system due to a leaky cylinder
4) Flexi-hoses are way past their sell-by date.
5) air in the system due to a faulty master cylinder
6) Faulty m/c
Regarding #1 it is not easy to bleed a Minor. The rear bleed nipples do not bleed the cylinders, they only bleed the pipe and connector, therefore you need to 'prime' the rear cylinders by removing the brakes (or at least slacken the adjuster) allowing the piston to move when under pressure. This movement allows fluid into the cylinder and when the piston is pushed back the air at the top of the cylinder will be pushed back into the pipe leaving fluid behind. This can need to be repeated a fair few times when fitting a new cylinder.
The rear connector banjo should be aimed upwards to allow the air to collect under the bleed nipple, which helps with bleeding out the last remaining air.
You have to remove the m/c to inspect it. I'd suggest not to risk wasting money on a seal kit unless you have already inspected the m/c.
If the bore is rusty around the secondary seal (the one nearest the pedal), a seal kit won't fix it.
If the bore is fine, then fit a new seal kit anyway (but only if the bore is fine).
I'd double check 1 to 5 before removing the m/c (unless you can see bubbles rising, then it is best to do the m/c first)
Removing the m/c is not too bad when you're used to it. Most of it is overcome by determination!
Here's the best way I've found:
A) remove the brake pipe from the T piece on the rear axle.
B) stick a container under the end of this pipe and catch the fluid from the m/c. Often it will empty itself (so have the container nearby)but that does imply a fault! If it doesn't pour out by itself then pump the fluid out - this is better than splilling it everywhere.
C) Remove the brake pipe from the front brakes at the connector by the side of the chassis leg
D) Using a strong bar (I use a 2 foot long wrecking bar) and a thick piece of wood on the floorpan (about 20" long - or more if it fits) check that you have the strength to lever the torsion bar away from the floor by about an inch. It's not easy so if you can't do that see if you can lever it with a jack on the end of the bar. The bar needs to be positioned just to the side of a bolt (or in the middle of both but getting it moved enough for both bolts in one go requires more effort)
E) when you are happy that you can move the torsion bar, take the nuts off the m/c bolts
F) using a screwdriver / thin diameter bar and small hammer, tap the bolt head up to the torsion bar. Then bend the torsion bar just enough to clear the head and
push the bolt throught the chassis leg from the far side [DONT GRAB THE BOLT HEAD WITH YOUR FINGERS UNTIL IT IS WELL CLEAR OF THE TORSION BAR just in case something slips - fingers are too precious to take any risk]
G) Push the master cylinder back into the chassis leg until it is clear of the brake pedal pushrod.
H) Lift the front of the m/c up above the pushrod and slide the m/c forwards and out of the leg
I) Disconnect the rear brake pipe from the back of the m/c when it is nice and easy to get to
(If you try and disconnect this one with the m/c in the leg it can take ages)
To get into the guts of the m/c remove the circlip with any old circlip pliers and then also the big washer (it is spring loaded so you may need to press the piston into the m/c using a 3rd hand to make it easier)
The piston will normally remove itself with the spring force.
If you can get that far, I'm happy to give further tips on inspecting the seals. The cup seal and outlet seal should be changed if the m/c seems ok and these will come in the seal kit anyway
Re assembly is 90% a reverse of the previous instructions, however you need to take great care when reassembling the m/c seals. It is often best to remove the pushrod from the pedal (not highly recommended) to get the boot to fit the pushrod and m/c correctly however due to the pedal return spring this is a prize pain and not good for a novice!
The seal kit I got (from Bull Motif) 2 weeks ago was pretty awful. The output seal was the wrong size and I wasted 2 hours getting it to the right size so it would assemble
The boot didn't have a drain hole! (i cut my own hole) and the boot was a bad fit meaning the whole thing was a paint to reassemble, and not even worth trying to get it to fit the pushrod properly.
Oh yes...if the brake pipes refuse to screw back into the connectors, then that means the end of the flare nut has been flared (the nut shouldn't get deformed but it can happen usually from being overtightened). The bodge way to cure it is to file down the end of the flare nut (highly not recommended) and the correct way to fix it is to replace the pipe so that you have new flare nuts.
If the pipes are old anyway it is often worth just to replace them - then you don't have to worry if the pipe is damaged when you unscrew the flare nut (often the pipe rusts into the nut so it turns round with the nut and wrecks itself).
New pipes are cheap, so it saves a lot of aggro.
Good luck!
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