Bleeding the bleeding brakes !

Technical Big Block Tonti and Loop Frame models
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1

So having spent ~6 hours trying to bleed the linked brakes i have decided to take a break and seek the knowledge on here. Le Mans 1 with fwd and rear pipe manifold splitter only, no proportional valve.

So here is what I have done …

1 Bled the front using a vacuum bleeding tool, nipples both side.

2 Then bled the rear.

3 Had an issue with suspected air being drawn in via the nipple threads so I greased these.

4 Repeated the front - then the rear, etc ad nauseam.

5 No matter how many times I have tried the lever refuses to pressurise the system.

Before assembly I have fully serviced both calipers with new seals ring, besides if there was an issue here I would have fluid leaking from behind th pads and dripping down the wheel / brake disc. All four pads / pistons are fine.

I serviced the master cylinder with a new piston & seal kit. I pre-greased this with the brembo assembly silicone grease and all went together smoothly and immaculately clean after a session in the ultrasonic parts washer.

I’m using DOT4 fluid, which is quite thin.

So I have now dismantled the master again. It and the pistons, old & new, have been back through the cleaner.

The barrel of the master is clean and shiny, and no scratches that either my wife or daughter can feel with their fingers (my sausages ar too fat !). I can see a grey area to the top of the barrel that might be a small depression about 10mm before the reservoir hole, and about 10mm in from the front diameter step so about where the rear lip seal would sit.

I intend to try another re-assembly and repeat the bleeding exercise.

I have read that using a syringe to push the fluid up to the reservoir is more effective, so i will try that. What volume syringe do I need to fill a caliper and purge to the cylinder in one go ?

Thanks for reading.

2

Try adjusting the rod from the lever to the master cylinder so that it moves the piston further,
(had the same problem and this worked for me) :+1:

3

Thanks Phil - I will try that. I have just ordered a large syringe.

4

Hi, one of the main issues with bleeding the ‘linked’ brakes is that you are basically trying to bleed an upside down ‘U’ and as we all know air rises!

I use two methods that seem to work for me. Firstly you can remove the Calipers and hold them above the M/Cylinder and then bleed in the normal fashion. It can get a bit messy but it works. I usually put something inbetween the Pads roughly the same thickness as the Disc.

Secondly, you can use a syringe. i have a 60ml one which seems about right. Back Bleed by putting the syringe over the bleed nipple and forcing the fluid back into the M/cylinder expelling the air. Be careful, dont push to hard else the Brake fluid will shoot everywhere when it reaches the Reservoir.

Hope this helps, Nik

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5

It sounds like you are managing to pull fluid through with the vacuum pump so the master cylinder is open to the reservoir.
As mentioned above, try elevating the calipers to help any air rise.
The other thing I find is that i always need to pump the pedal vigorously for while to build up some pressure enough to push the caliper pistons out onto the discs.
Some people say that tieing the pedal down overnight also helps.

6

Well, that “overnight tieing” will defo help with front master,as you open the way for the air bubbles to raise up and beyond the master piston. I did it and it works. But in case of linked setup, this theoretically could happen only with rear caliper, as front one’s pipe , as mentioned by Nik, forms upside-down “U”, so pushing/pulling the fluid will be necessary.

7

I have re-reassembled the rear master cylinder and paid closer attention to the small clearance between the operating cam lever and piston end face. I have also put some assembly grease on this.

I have lengthened the brake con rod to its maximum.

The brake is now working following the syringe back flush method, but seem like a lot of movement.

I need to take it to an MOT station brake test machine to properly check it out. The bike itself is MOT exempt.

2 Likes
8

I have received a length of aluminium tube and have reamed this to 5/8" to make a new liner, and am now polishing this with oil and a thorough spin on the static reamer to hone the surface. Certainly producing plenty of blackened oil. The master piston now enters but is a little too tight so a bit more polishing required.

The difficult bit will be machining the master cylinder bore reasonably concentric and precisely to accommodate a light sliding fit of the new cylinder liner. Once this is achieved (noting a new master might be required !) I will lock the liner with Loctite bearing fit, then drill the two port holes. I will need to carefully remove the burrs from the holes exit. and will further polish with the reamer to achieve this.

9

I used a plastic bottle, the thick type you buy not a pop bottle. Used two bicycle inner tube valves. The outlet at the bottom of the bottle without the valve core and the other screwed into the cap.
A plastic tree pot hooked up takes the bottle and a bicycle pump provides gentle pressure to push the fluid the same way as returning pads would.
I’ve been using this set up for many years, it seems to work on everything. The valves cut from bike inner tubes have rubber surrounds to seal them and are small enough to take clear bleed tube. Use a hypodermic syringe to suck excess fluid from the reservoirs if necessary, purchase from local farm shop, big 20cc for cows.
You can buy a pressure bleeder that does the bottle bit :grinning:but £££££

Shifty