[MDairheads] Brakes / Hydraulic hoses. An inquiery

[Sailorcto]

Hi Bill,

I can’t recall the year of your RS, but if that is the bike you are referring to, I think it is post-1980. So, assuming these are Brembo calipers, not earlier ATE brakes...

When you mention that the calipers were fully reconditioned, did that include splitting the halves, removing the black square nylon piston ring and carefully inspecting the square groove it retracts into (a dental mirror with a flexible head was the best inspection tool that I’ve found)? If not, it would be worth doing IMO. The memory of that black nylon ring returning to its seat is what causes the piston halves to retreat upon release of the lever on a Brembo caliper. I’ve found many of tiny aluminum oxidation pimples in that groove to prevent the ring from fully seating as you describe. That groove, while tough to get to, should be surgically clean and completely free of aluminum acne if-you-will. Be very careful not to score the walls of the cylinder that the pistons glide across. Its the groove that deforms, preventing the ring from easily retreating to its fully seated position.

As for the rubber hoses, they rot and swell from the inside as they age. It could also be that the swelling affects (absorbs) some of that memory too. Squeezing the brake is a forcing action so I would expect higher pressure than return. Any air leaks or other disruption in the hydraulic system when the lever is released will affect the gentle retreating of the caliper pistons and cause what you describe. If it is an air leak, they would eventually return the pistons to their seat and release the grip on the disc. If no air leaks and no nylon ring/groove obstruction—its likely the pins and/or the tired hoses.

Replacing the pins that the brake pads ride on is also worth doing and cheap. Despite looking ok, the pads can hang up on those old pins and even slight friction can disrupt the returning action. These are often over looked. Over aggressive cleaning up the pistons can also remove the Teflon-like coating and affect the smooth movement of the return path of that the nylon ring re-seating.

Lastly, when splitting that caliper there is a tiny o-ring where fluid passing between the halves. That should be replaced too with one from the correct compound (I can’t recall it right now, otherwise that could also compromise the closed hydraulic loop and allow air in spoiling that retreating process of that black, square piston ring. 

My $0.02 cents and ramble.

—Tony B.

On Mar 26, 2019, at 20:40, Bill Lambert via MDAirheads <mdairheads at casano.com> wrote:

 Gentleman, Ladies,

 In recent efforts to sort out nagging brake issues, I have a question.  What causes old rubber brake fluid hoses to stop allowing fluid to return to the master cylinder reservoir upon releasing the brake lever ?  With the caliper and the master cylinder fully reconditioned, and new fluid pumped in, the brake pads only partially retract .  A small parentage of drag or squeeze remains. The wheel refuses to spin freely.  When the old hoses are tossed in the trash and new hoses are installed, compliance is restored.   I don’t get it. 

 With 120psi blasted through the old hose, no obstruction is witnessed.  When the rubber hose is razor knifed into short sections, no crimp, kink or obstruction is obvious.  The ID appears consistent.  The old hose acts like a one-way check-valve.  Pressure is squeezed in, but fails to escape. 

 Any ideas.

  Bill.  
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