Many brake systems can be difficult to bleed

The best way to deal with this sort of issue is to reverse bleed them
Instead of of pumping the fluid down from the master cylinder get situated to bleed from the calipers up.
It can be done with a brake fluid pump or by raising the fluid supply up above the master cylinder and using a long tube to supply the fluid to the caliper bleeds. A pump is by far the quickest way and they are not overly expensive.

Here is an example of one such pump



I'm sure you can find a similar one from any good automotive supply shop down your way

a big syringe would work fine too and could be used also when changing the fork oil.
http://www.amazon.com/Motion-Pro-Fork-L ... 912&sr=1-5
If that doesn`t work you should try another brake pump. The pump squishes out fluid to the top also when it`s not ok or the seals would be put in the wrong way as that doesn`t require a lot of pressure. This doesn`t have to mean it actually puts full pressure on the system.

G.

PS: had you touched the lever adjusting screw ?

Hi all,

I normally use a syringe but the one-way valve on the speed bleeders prevents this.

I probably did touch the adjusting screw before the overhaul to try and get a bit more action at the lever. Should it be in a particular position?

G, your remark about the master cylinder pressure makes me think about the direction of the primary seal in it. The instruction sheet helpfully tells me to mount it with 'the lip facing the interior of the cylinder'. So: does that mean the 'open' end (the part with the largest diameter) of the seal is facing towards the banjo bolt on the master cylinder or away from the banjo?

One consolation: if it IS the the wrong way round I wasn't the only one fooled by the Italian instructions...

Thanks for not giving up on this cause (if not a lost cause, then at least a wandering one)!

Mike

Mike,

I had something like this when changing the F08 caliper and installing a master cylinder rebuild kit on a Moto Guzzi; beld and beld the thing till no bubbles, but the lever was still really soft. The brakes did work but the lever was hopeless. Now, being that the Guzzi had intergrated braking and the foot brake operated the left front disk and the rear disk I went out for an experimental ride (as still had 2/3 braking available).
Anyway, in 5 minutes the front brake lever (which did only the right front disk)was good and firm. Only conclusion was that some vibration/bumps etc pushed air through the system into the master cylinder.
Is it possible for you to safely go up and down your street a few times, assuming there is some braking available even if it is difficult to feel?

Cheers,

Fraser.

just a quetion.. you are SURE that the rebuild kit was EXACTLY the same?

Hi all,

OK G, seeing your reply I thought that the seal was mounted the wrong way round. Hooray! Easy to fix. Took the master cylinder off and took the internals back out. Turned the seal around and put it all back in the master cylinder.

After I topped up with brake fluid I tried the lever: disaster! The lever goes in but does not come out. Pulling out the lever actually seems to suck brake fluid back.

Here's how I put it together (the master cylinder is in the foreground so squeezing the lever would move the piston towards you): Master_parts1.JPG [257.26 KiB]
Downloaded 771 times

and from a different angle (the master cylinder is on the left): Master_parts2a.jpg [172.01 KiB]
Downloaded 771 times

So either everything was in the right order in the first place (but didn't build any pressure) or something else is going on.

This thing really has got me almost beat.

Nicola: the kit is exactly the same as what was in the cylinder originally.

Fraser: I have no brake pressure at all so I would hesitate to ride it anywhere.

Thanks again for all your advice guys!

Mike

If the master cylinder is very worn, then a new seal can make things worse. I have had this happen before on a clutch, originally was soggy, new seal kit - result no clutch at all. Solution was a new master cylinder.


As far as bleeding, I have always had success with the following method which requires no special tools other than 3 hands.

1. Fill the system with fluid by pumping with the bleed valves open until fluid comes out
2. Close the valves and pump
3. Hold lever in whilst releasing bleed valve
4. Close bleed valve and pump again
5. Repeat 3 & 4 until success
6. If no success, pump system and tie lever aginst the bar over night
7. Repeat 3 & 4 until success

Never had a failure with the above procedure.

For safety best have a tube over the bleed valve venting into a suitable container as otherwise may get brakefluid everywhere as it will be under pressure.

I suppose a worn master cylinder is quite possible. If I replace it I would like to start upgrading to newer parts as I have been planning for some time.

Would a 916 or 996 master cylinder work with the original F08 calipers?

Mike

Mike,

If you need a new master cylinder Bevel Heaven have them for USD$115.
http://shop.bevelheaven.com/detail.aspx?ID=206

Cheers,

Fraser.

what about that o ring, shouldn´t that sit on the piston, in the groove behind the seal ?

G.

G,

The inner diameter of the O-ring is about the same as the outside one of the piston. When I took the cylinder apart it seemed to sit on the recess furthest into the master cylinder. Do you think that is incorrect?

I will check what the drawing on the Brembo instructions shows (if anything).

I'm going to have another go at putting all this together but I'm also considering alternatives. Do you know if a 916/996 master would be able to operate the F08 calipers? I would like to upgrade the brakes to goldlines at some stage. If I'm replacing the cylinder I may as well buy the one I want to end up with.

Cheers,

Mike

Mike,

I`ll look through my parts boxes right now and if possible disassemble one pump today to let you know.

Of course other pumps will work. Generally.
The smaller the piston diameter the more aggressive the pump usually is.
Bigger pumps like a PS16 will need more hand force to create the same pressure but this will give the rider a better feeling for the brake.
The question may be the relation between brake pump piston diameter and brake caliper piston diameter.
4 or 6 piston calipers have pistons with different diameter. The smaller pistons react sonner on pressure than the slightly larger ones which makes the brake more sensitive.
4 piston calipers usually need a pump PS16 or bigger. The caliper pistons are smaller than the ones of the 2 piston Paso calipers.

What I`m trying to say is that I don`t know if there`s a kind of "gear ratio".
It may be that a caliper with bigger pistons may need a brake pump with smaller piston. Anyone who knows ?!

I`ll look for the brake pump now ...

G.