Advice for fixing "spongy" front master cylinder after attaching and bleeding ?

[Guest]

I just replaced my front master cylinder on my '79 suzuki gs850G.

The Chinese knock-off it replaced was TOO pressurized (1/2 travel to hard lock).

I purchased a used front master cylinder from a member here. I attached it to the bike, filled with fluid, pumped a few times with cover off to get bubbles out, then bled the brakes several times on each side (my bike has dual front disk brakes).

The brakes will bite and seem to stop the bike and make the front forks dive but the lever still feels spongy and I can (with enough force) pull the lever to the bar.

I left the lever tied back to the bar over night to no effect.

Can anyone suggest anything ?


sean

[fastbysuzuki]

I just replaced my front master cylinder on my '79 suzuki gs850G.

The Chinese knock-off it replaced was TOO pressurized (1/2 travel to hard lock).

I purchased a used front master cylinder from a member here. I attached it to the bike, filled with fluid, pumped a few times with cover off to get bubbles out, then bled the brakes several times on each side (my bike has dual front disk brakes).

just wondering about your quote " pumped a few times with the cover off"
i take it that when you you bled the brakes you had the cover off all the time to replace the fluid lost in the bleeding process?
are you getting a decent flow of fluid through the bleed nipple when you pull the lever back?
you should not be able to pull the lever back to the bar whatever so there must be air still in the system or the cylinder is not delivering enough flow to push the Pistons forward enough or the brake hoses are expanding (through old age) assuming they are rubber ones.
I have bled the brakes as you are doing with success. But now use a hand vacuum pump to bleed my brakes.

[Nessism]

What is the piston size of the new/used master?

[seanarthurmachado]

What is the piston size of the new/used master?

The new chinese knock off was 16mm. (I have another thread on that one.) It had great pressure, but was too great.

the used one, I do not know.

sean

[seanarthurmachado]

just wondering about your quote " pumped a few times with the cover off"
i take it that when you you bled the brakes you had the cover off all the time to replace the fluid lost in the bleeding process?

No, I pumped a few times, pressure built up, then I filled up, covered the master cylinder, tightened the screws, and pumped some more.

I ran out of fluid once and started over. I get fluid out into the bleeder bottle with the kit.

saw bubbles at first then believed them to be gone.

[ShadowFocus603]

If you bleed your front brakes properly using the bleed screws on the calipers that will eliminate a lot of your issues. Second, once you have them bled zip tie the brake lever as far into the throw as possible and leave it for the night. This will release the bubbles and air you can't get otherwise and give you nice firm brakes.

[seanarthurmachado]

OK I checked the 2 OEM master cylinder units.

The one that came on the bike shows "14" then "7 A 3" (I assume that means the master cylinder is 14mm piston ?)

(Ive read I should have a 15.xxx size piston in the master cylinder)

The used one I just bought shows "1/2" then "9 N 3" (this was from a gs that had buck horn bars and is slightly tilted on my straight bars.)

Does anyone recognize those numbers ?

Sean

[seanarthurmachado]

If you bleed your front brakes properly using the bleed screws on the calipers that will eliminate a lot of your issues. Second, once you have them bled zip tie the brake lever as far into the throw as possible and leave it for the night. This will release the bubbles and air you can't get otherwise and give you nice firm brakes.

I did all that. (pumped the master cylinder to bleed from the MC, then moved down to left front,bled from the front left caliper, then to the front right caliper.)

Tried to tie down overnight but it did not help.

sean

[Nessism]

With a 1/2" bore master there will be a soft lever feel. Nothing you can do about that. The proper master is 5/8" bore size.

[seanarthurmachado]

With a 1/2" bore master there will be a soft lever feel. Nothing you can do about that. The proper master is 5/8" bore size.

OK so sounds like this master cylinder is the wrong size then. I didnt notice the numbers when I purchased it.

SO silly question, since I think the 16mm bore is too hard, does that mean I could "customize" the pressure by finding a 15mm bore ?



sean

[exdirtbiker]

Before you spend more money on parts, borrow ( or buy) a vacuum bleeder. ($20 version works, but $30 one is worth it if you work on your car brakes yourself)
I threw $400 in parts ( rebuild master, new master, new slave) at my VT750C hydraulic clutch before I realized you can NEVER bleed the air from the hose connection at master cyl. It's too much of a bend "up". After I put the vacuum bleeder on it, I was on the road in 20 minutes.
Sometimes it's that simple.

[seanarthurmachado]

Looks like this is fixed finally. I ended up buying another master cylinder from ebay (a used model from a gs850 80-81, with square plastic container.)
this model is a 5/8 inch size piston. after attaching, bleeding the brake master cylinder, then bleeding the brakes the lever goes about 1/2 way to hard lock. More gradual, not the "full off/full on" I had from the chinese knock off.

sean