Manky Monkey Motors

hi all , this could turn out to be a no brainer but i bought a set of custom levers , clutch and brake , after several probs with the front brake ( caliper flexing ) i thought i had a good brake but the day before the msva test i pulled the front brake and there was nothing , the lever pulled back to the bars with no effort at all , on the next pull perfect brake again .
i know its not the calliper or the brake line and i bled the system using a 1 meter length of clear hose and no air showing also i did compress the hose and pulled the lever and there was no movement of the lever when the clamp was released .

so im thinking that my custom made master cylinder is goosed but am open to suggestions , ive never opened one up before so this is new ground to me

cheers for any advice

Blair, check out the last few posts of my Pop build thread in Hotrod Tech. We've had very similar problems with the truck, which turned out to be the master cylinder having too small a bore.

MAYBE - if its a new brake lever but original master cylinder, maybe the heel of the lever (the bit that actually pushes the master cylinder piston) isn't making contact? Maybe the pivot point of the lever isn't a good fit, which could mean lack of contact with the MC piston.

Sounds a bit unusual for the seals to have given up the ghost in the MC (so the psiton is physically moving as it should be, but the seals aren't moving the fluid), but its a possibility.

OR maybe the pads have returned so far into the caliper (no idea why), that activating the brake lever is moving the pads, but not far enough.

Does the fluid level drop spightly when the brake is activated, and then return?

Anyone else with any ideas.......................

Try this, remove the caliper and place a bit of wood similar thickness to the disc between the pads,with the caliper in your hand (upside down) you should be able to see the pads grip,then release as you let go of the lever, repeat many times to establish the caliper pistons move freely in BOTH directions.
Then try to remove the wood with brake applied, if you can remove the wood and feel pulsing on the lever then the m/c sealsare not holding pressure
Try and see that the lever heel is making good contact with the m/c pushrod (operating rod), pushing  and staying in place,(not letting the pushrod slip backward off a rounded corner on the heel )
No sneaky fliud leaks behind dust covers ?
Last, as the brake is applied, feel the brake lines to see if they pulse/ swell,if so some of the hydraulic action is swelling the pipe rather than pushing pistons so they are spanked
If the above checks reveal no horrors then its master cylinder shopping time

PS if its a custom jobbie is there a custom lever that goes with it ?

First bit of advice......Don`t open up the master cylinder, if it is faulty it may affect the warranty.

What size caliper are you using?
Is it a single or twin piston?
What size is the master cylinder?

sorry guys , been a bit busy

sorrt Tbone to late had alreadyopened it up , took it round to a seal specialist that we sometimes deal with and it looks like the maine seal had a small tare/cut in it , they are going to make a couple of new one for me, before i took this course of action i tried several different calipers but all did the same , i also changed out the brake lines and no difference .

Fatboy , we tried a few different things and it all seam to point to the seal . yest there was a custom clutch and brake lever that were made , i may have managed to find the lad that made them , i will let you know later

cheers lads
 

Glad to hear you getting somewhere. Its easy to wreck a seal if  you are a bit kack handed fitting it.
I spent a day fitting a clutch slave cyl on an Audi,bastard of a job, bastard to bleed up but got it done,had a cup of tea an a fag, went to go fer a drive, Hey Presto....... brand new slave cylinder failed in 15 mins
Put that down to poor assembly,got a refund no probs
Brand new dont always mean works !

well the saga kinda continues ,

the chap i know managed to get seals remanufactured for me from the original samples , had to take a min qty off 3 each, caost £ 42.00 . came home and fitted the first set of seals and could hardly move the piston in or out , now im a rubber man for my work so you would think i would have know to not put a small smidge of grease onto an epdm seal but i did , started great . set up a spare caliper and disc on the bench . bled the system and brilliant so i left it and over the next couple of days and everythiing seamed fine , so i stripped it all down and reset it up with the proper caliper but when i went to bleed it the piston was solid and would not return ---bugger

after a bit of thought i managed to get the piston back out and the seals has expanded exponentianly to the grease i had used to lubricate them . so second set fitted and this time i used a little veg oil and the piston slid in and out with minimal effort but on bleeding the set up i still had no brake after 1/2 a ltr and was still pulling air through so i striped it down to descover that there was a small nick out of the front seal -----bugger didint spot it going on but maybe i  did it , who knows

third and final set fitted , no veg oil but tried the piston in and seemed to be free enough , but after another 1/2 ltr and still pulling air through , bugger . bugger . bugger .

then brainwave fitted a syringe filled with brake fluid and reverse bled it . after a second a massive burp from the resivior and i had pressure ,must have been an air pocket . i would say that although i had used 50 gallons of new fluid the stuff that came up through the resivoir looked dirty but fitted it to the bike and no problems so far

no bugger  ;D

Erm, think that's progress ...isn't it?

think so Mr MM , not as much as a runaround as your truck system and if needed i could just splash the cash and buy a new master cylinder but i really like the ones ive got  :P