'81 Interstate replacement front master cylinder / spongy lever feel?

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DeansZG

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The front master cylinder on my '81 Interstate started leaking & instead of rebuilding it, because of the scuzzy reservoir & the overpriced re-build kits, I bought a brand new m/c off of flea-bay. The seller listed it as a direct replacement for "80-'81 goldwings. Got it mounted up, bled it, pressurized w/ a nice & firm lever.....

Now comes the fun stuff... connected all the lines, going thru the prcedure of evacuating the air from the lines & have gone thru damn near a pint of brake fluid trying to get a firm lever again to no avail. :head bang:
Used a mity-vac to help eliminate all air from the lines & tried the rubber-band method of pulling the lever & leaving it set overnite also.
The lines look to be in good shape, w/ no swelling & might have been replaced w/in the last 5 yrs. or so...

The bore on the new mastercylinder is 2 mm smaller than the oem ( 14mm vs. 16mm ). Could this be the reason for no lever firmness??? I'm at my wits end trying to solve this....

Thanx in advance...
 
yes ..maybe so but on segle piston calipers like you have take the bottom bolt out of caliper mounts so thats the bleed screws is at higest possition and still on rotor and re bleed there ... full mounted they can trap air there in the piston itself
 
Well I'm not sure in your case but I modded in a China made rear mc on my 83 which was 2 mm smaller and I couldn't get good pressure.
I'm not sure though if it was defective or the piston size was the problem.
The mc was new, the bore smooth and rubber looked good.
I've since modded in a 1800 rear and it has good pedal travel and firm.
I haven't measured the 1800 piston size or length to compare with OEM but the 1800 rear brake is also linked with front like the 83 so I'm chocking it up to the China mc being too small to push enough fluid.
 
I am running a 12.7mm CBR1000f MC on the Rats Nest and that gives me more pressure than the original larger bore MC.
 
Might want to check that criss over mount on the front girl bracket... Make sure it is clean and clear... also.. buy replacement crush washers just to make sure they all are good and sealing well.. the caliper bolt to method does work well.. and I heard that when I was on the nag-sab board... they mentioned the same thing so there is something to it.
 
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