Rear Brake failure

[ArthurFarrow]

Bought a Hesketh a couple of weeks ago and joined the club. Hello will be at Stafford so Ill roll up on the H stand.

Well finally the weather turned and I finished all the detail work I needed on the bike (seat is a pig to open) however 10 miles into the run the rear brake totally failed. No evidence of leakage anywhere so crept home gently (or as gently as a 3,000 rpm tickover allows-mayeb I caught the cable when the tank was off) Not fun!

My suspicion is a burst seal on the master cylinder as the rear calliper was not that hot and seemingly not binding. Any idea part number ? I traced the calliper seal set to 98.5032.50 ...anybody confirm that is right?

2 hours after my ride and the brake is starting to have some feel again so maybe it is the rear calliper..any thoughts folks I hate working on brakes.

[Rob B]

Hi Arthur. If the rear brake was working perfectly and suddenly stopped working with no drop in fluid level or visible seal leakage, it won't be a calliper seal. If it has original brake hoses then look for a bubble in the outer jacket. Otherwise, yes, master cylinder seal is leaking back. Don't be concerned about working on these brakes because they are the simplest and best available, in 1980! All spares available from Guzzi bits, Ducati or Laverda etc. Front master cylinders becoming rare now.

If you do still have the original hoses then I would throw them in the furthest bin you can find. They will be knackered and they will let you down, possibly badly! Nice black braided replacements available from Hell for example.

[Chris N]

Hi Arther.
the way i open the seat (i could be wrong) is with the key turnd i pull strait back with your fingers under the rear light unit.
when i sorted the carbs on my hesketh recently i found getting the throttle cables to seat properly in the carb slides and into the two to one adapters was a bit of a faf due to the sharp bends and differing cable lengths,it took a couple of goes to get it right.
with the brakes one of the first things i did was get 3 caliper seal kits from www.gutsibits.co.uk  part number BRA37234 and using the excellent https://opimedia.azureedge.net/-/media/Images/MCC/Editorial/Articles/Magazine-Articles/2011/09-01/Brembo-Brake-Caliper-Rebuild/Brembo-Brakes---rebuilding-guide-pdf.pdf?la=en&hash=A15D9ADF296472786168DF1066DB48F1F89FF235 it was a doddle.i also changed to silicone brake fluid.your fault sounds like a pipe expanding under pressure issue so changing them as Rob said would be the best idea.
How do you find the gear change from first thru neutral into second?
Chris

[ArthurFarrow]

Yes the pipes were nicely marked 1981, still no leakage and am sure the master cyl has gone pop internally. Probably a master cyl seal "picked up".

I ordered a black coated braided line from Hell. 86 cm I think with 20 degree fittings either end. I have previously used Venhill on my Morini but they have a different style of manufacture and the banjo screws onto the pipe.  So I am going to use a single line and a banjo bolt incorporating a brake pressure switch.

I got a new rear calliper from Gutzibits which can stay on the shelf if I do not sue it. Probably will use it rather than rebuild the old one which I would do anyway cos I can. New master cylinder seals from Claire's Hesketh bits and also bought the tool.

I will use Castrol Dot 4 fluid....Chris N you used Silicone fluid? Is that not Dot 5 and not for this type of brake??

I will move on to do the front ones and the clutch shortly...anybody got the clutch length??

Gear change seems OK , but I only did 10 miles before the rear brake failed. Missed gears a couple of times but I have many vintage bikes and am used to "feel" when changing gear.

Thanks for advice so far

[Chris N]

HI there is a big debate about silicone brake fluid and dot specs,personally I have used silicone brake fluid in all my bikes for a while now with no braking problems.not absorbing water and not ruining paint are the main reasons i switched,also the harley fluid is an interesting purple color.

[ArthurFarrow]

Well I rebuilt the master cylinder ( carefully) fitted new Hel cable and a brand new rear calliper.

Nothing

Tried reverse bleeding, normal bleeding, even shouting at it...all bleeding useless not a vestigial of anything and I'm close to building a huge funeral pyre and burning the bloody thing

Any ideas anybody ? I ordered a Mityvac in desperation.

Please god dont make me ride my BMW all summer........

[Chris N]

Hi Arthur the Mityvac should do the trick,i always use that to pull fluid thru the system then go i go back to opening and closing with a spanner to finish.

[Dave Snr]

Hi Arthur,

Are you getting any fluid to flow out of the master cylinder?
Disconnect the hose connection at the master cylinder and see if it pumps fluid out. If not check the holes connecting the reservoir to the piston cylinder - one of them is very small and easliy blocked with dirt or the grease they give you to lube the new seals. Check the piston is returning fully. Once the master cylinder is pumping fluid, you can reconnect it and it should bleed OK.

Good luck
Dave

[Rob B]

Hi Arthur,

I agree with Dave, having made the same discovery. The second orifice for the fluid return is indeed minuscule. Probably 0.25mm so very hard to spot but the Master Cylinder simply won't do anything unless this is clear...