Front Brake Problem

[Warbucks]

I've just fitted a brand new master cylinder which appears to be working properly before it's mounted to the handle bars. I can depress the piston and it will return freely. I pumped it easily with my thumb and it blew a heap of air out of the bleed nipple. That's good.

However, when mounted and the brake lever is pulled, it feels very stiff. Furthermore, the piston will not return to its original position when I release the brake lever. That renders the brake lever absolutely useless. The only way I can get the piston to return is to take the master cylinder out of the mount. When I do that, it seems to be working as it should.

What am I doing wrong?

[montmil]

Have you filled the reservoir with brake fluid and bleed the system or are you operating it dry?

I'm not clear if you are describing a binding issue with the master cylinder's piston or...?

Did you lube the piston with brake fluid? Confirm brake line is not clogged? Brake caliper operation as it should?

[Tony Smith]

Take the master cylinder off again and look at the bore in the lever perch that the master cylinder runs in. I will bet that you have left the old rubber grommit and or the old sleeve washer in there.


Leaving the old parts in place and fitting the new ones to the master cylinder will do what you describe every time.


Outside chance, with the master cylinder removed, check your lever for full, free movement.

[Warbucks]

The reservoir is full. The master cylinder is brand new and was pre-assembled. Everything on the master cylinder is brand new. It was delivered as a complete assembled unit. The master cylinder is lubricated. All I had to do was mount it on the hand grip and connect the brake line. There are no blockages in the brake lines. I ran a heap of compressed air through the lines and both halves of the calliper and it was all proven to be clear. I even installed a new kit into the calliper.

If I dismount the master cylinder from the hand grip mount, I can easily operate the plunger on the master cylinder with my thumb (instead of using the brake lever). It goes in and out easily like it should and I can bleed air from the system.

The problem starts when the master cylinder is bolted to the hand grip mount. If I squeeze the brake lever it will push the plunger in but it's very stiff. Once I do that the plunger will not return. Therefore I'm left with the brake lever having absolutely no pressure on it at all. The plunger seems to be jamming in the hand grip mount only when the master cylinder is bolted on. I can't understand why it would do that because the hole that the plunger goes through has a rubber grommet in it. I even tried taking the grommet out so that the hole was heaps larger than the plunger. It still jammed. The grommet is now back in.

There are two washers on the brake lever end of the plunger. The outer washer can be moved and it will come off if I move it out far enough. I'm assuming that washer needs to be up against the other washer at the master cylinder before bolting it on.

[Tony Smith]

Ok, last try from me before I admit being stumped.

Was the old master cylinder a round or square reservoir type? I have always understood they interchanged but I've never done it myself.

With the new master cylinder off place the old one next to it and find out which dimension is different is my last suggestion - it may be that you need to relieve some small amount of metal due to a small dimensional difference.

[Warbucks]

The master cylinder and reservoir are identical to the one that I took off. They are stock standard 12mm cylinders. The reservoirs are the square type.

I'm a wee bit closer to solving the problem. In my last post I mentioned that there are two washers on the plunger at the brake lever end of the master cylinder. The inner washer sits against the cylinder. The outer washer has a rubber bush which makes it sit snug on the plunger. That's the washer that I mentioned that I could take completely off.

As an experiment, I removed that washer and left the inner washer on the plunger. I then bolted the master cylinder to the mount and refitted the brake lever. That works! The brake line visibly pulsates when I pull on the lever. On releasing the leaver the plunger now returns as it should. I now have the front brake working.

Now for that washer with the rubber bush. It was obviously the cause of the plunger not returning. It was on the master cylinder when freighted, so it must belong there. The old master cylinder that I tossed in the bin also has these two washers.

I've noticed when sliding that washer up and down the plunger its pretty hard to move. Could it be as simple as lubricating the rubber bush and the plunger with Vaseline or something like that so it can move freely??

[Barry]

I've noticed when sliding that washer up and down the plunger its pretty hard to move. Could it be as simple as lubricating the rubber bush and the plunger with Vaseline or something like that so it can move freely??

I use Vaseline for all sorts of things including lubing rubber O rings but often wonder if it's suitable for all rubber types with it being a petroleum based product.  I imagine the washer is the type of rubber to cope with brake fluid so maybe brake grease (red rubber grease) would be better if you have any.

[Warbucks]

Problem solved! There was a small sachet of grease included with the calliper kit, which I also used to lubricate both seals. I also used it to grease the plunger on the master cylinder and it now travels freely.

A line of text included with the master cylinder reading "Before installation lubricate the rubber bush on the washer" would have saved me about four hours trying to figure it out. The kits do not come with any instructions at all. I guess a brake guy would already know that, so I've learned something new today.

The outer washer with the rubber bush is there to prevent the inner washer bending under pressure. This was evident when I lubricated the plunger and rubber bush. After experimenting without the outer washer installed, the inner washer was bent from the centre out. I flattened it out with a persuader and put it all back together again. They're cheap crappy washers so it only took two hits to get it back to normal.

For those who are interested, to pass for registration; the law in Australia states that the brake lever needs to have at least 20% of its movement left when the brake is applied. Therefore, when you pull the lever through 80% of its travel the brake must be working at that point.

After reassembling everything I tapped the entire brake system all over with the handle of a screwdriver. If there is any air left in the system it should all float upwards. Sometimes air bubbles can form on the inside of the line and the fluid will pass beneath those bubbles. Tapping the line will dislodge such pockets of air. I left the cap off the reservoir so that the air (if any) can float up there overnight. Tomorrow I'll top up the fluid and bleed the system once more. The brake feels pretty good at the moment. It may get even better.

Just a word of advice if you have not played with disc brakes before. DO NOT ALLOW ANY BRAKE FLUID RESIDUE TO COME IN CONTACT WITH THE PADS OR THE DISC!

A friend of mine once helped me out doing the front disc brakes on a car I had. We bled the brakes and took it for a test drive. At the bottom of the driveway I gently hit the brake and then I heard a very loud bang.

Unbeknown to me, my friend did not wash his hands before he assembled his side. He merely wiped his hands with a rag, but there was residue still on his fingers. What was the bang? The passenger side locked up and the weight of the car ripped the calliper clean off. The pads were impregnated with fluid and in that state they could not be cleaned.

I got free beer for the night off my mate for that. Cool! LOL

 [smiley=beerchug.gif] 

[montmil]

Us men don't need no stankin' instructions, right? Glad you got it sorted.

Best way to purge the remaining air from the system is to tie back the brake lever and leave it overnight.