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Rear master cylinder issue

Julian Sudano

Tuned and Synch'ed
GT Contributor
Joined
Jul 25, 2021
Messages
61
Location
Los Angeles, CA
Just got my V65 SP delivered from Kansas on Wednesday. Was a bit disappointed to find the brake pedal did absolutely nothing to help stop the beast, on my initial ride. I guess that's the price you pay for buying sight-unseen, but I'm happy to have another project as it's been too long. I also wasn't getting the rear brake light when pushing the pedal.

I decided to try pumping fluid through the system as an initial experiment to see if it helped, as it couldn't get any worse, even if there was a little air left in the system. So I hooked up a clear bleed line to the rear caliper nipple, cracked the nipple and proceeded to pump with the foot brake. I managed to expel some dark, nasty looking brake fluid about an inch past the nipple, but whenever I released the pedal, it would simply suck it back in. I had topped up the fluid reservoir, but saw zero evidence of the fluid level dropping or mixing of dirty fluid with clear new fluid.

In frustration, I unscrewed the flexible line at the caliper, and when I disconnected it ,I barely got 2 drops of dirty fluid. Even lowering the end of the line below the master cylinder resulted in no more fluid coming out. I then disconnected the flexible line at the back of the m/c and there was a small amount of fluid, but that was it. I assume that's why I wasn't getting the brake light to work as there just wasn't enough fluid to activate it?

Next day I got a big syringe and attacked the front left caliper. I started but trying to pump fluid out of the system using the pedal - same result with nasty, dark fluid only moving an inch or so up the bleed line and then being sucked back in when the pedal was released. Moving on to the syringe, I tried to push fluid back to the m/c but all that happened was fluid peeing out of the nipple. I just couldn't get it past the caliper. Finally I disconnected the static line from the front caliper where it joins the back of the m/c and pushed clean fluid through using the syringe. Not much of the dirty fluid came though.....so a lot of air in there.

I then tried to push fluid from the rear caliper, using the syringe, but same issue - I just got massive pressure and couldn't get it past the m/c. Absolutely no movement of the fluid in the reservoir either. So I disconnected the fluid supply line from the reservoir, where it enters the m/c and it was full of fluid.

Why can't I get the m/c to suck in fluid from the reservoir, or push fluid past the m/c? Is this just due to too much air in the system. Would sticking pistons mean fluid wont make it past the calipers? The rear caliper does have the new anodized pistons, so it's had work done on the brakes at some stage. I'm wondering if the m/c was rebuilt incorrectly hence these issues...

Any input gladly received!
 
When you bleed the brakes, master cylinder full up, open the nipple at the caliper, press the pedal as far as it will go, nip the nipple back up so it doesn't draw fluid/air back (I think this is where youre going wrong) then release the pedal, then open the nipple, press the brake lever all the way then nip up the nipple, release the lever. Do this untill all air is expelled from the system and clear fluid comes out the bleed pipe, keep an eye on the master cylinder as if this goes too low you will have to start all over again. Press the lever and it will be firm. If not just repeat, should only take a few goes.
 
Mate - I tried that many times too, but still no luck! Even wrapped the nipples with a little teflon tape to stop air being sucked in via the threads. The thing was, at no time did the fluid level in the reservoir drop either so I'm wondering if there was ever any fluid in the master cylinder to begin with. Also, why wouldn't I be able to push fluid past the master cylinder with the syringe?

PS I grew up in Pakuranga.
 
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Sounds like the hole in the master cylinder is clogged. If the fluid was nasty, count on an overhaul of the cylinder or a new one. Plus check your caliper pistons as well. Other option is your rubber brake lines have delaminated internally and cause blockage. Generally that causes brakes to not release but who knows
 
This is typical of a master rebuild. When piston moves it puts fluid to brakes, when released it sucks fluid from reservoir into plunger cavity.
Take apart & rebuild. Kits are available.
If it doesn't work correctly the piston is plugged up w/dirt or old junk, it's not air. Fix it.

Look in the store for rebuild kit. -----15mm Brembo Pt#110.2739.20 Front is 13mm #110.2797.10
Same stuff for BigBlocks of that era. New master is around hundred bucks.
 
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Considering the age of the master, I doubt a rebuild kit would help even if available in the US. Haven't seen kits in some time due to lawyers and litigation. Contact Todd via the contact link in the Store to order a replacement.
 
They have kits & new masters available, I just got one for the Cal2, master kit.
They used it up to the CalVins in 09& 10
 
Considering the age of the master, I doubt a rebuild kit would help even if available in the US. Haven't seen kits in some time due to lawyers and litigation. Contact Todd via the contact link in the Store to order a replacement.
Managed to find one at MGcyles.com. I'm curious to see what the condition of the m/c is when I dismantle it....I may yet need a new unit. It looks like the rubber seal around the inlet elbow is not in very good shape, and those don't come with the rebuild kits.
 
Update:

I pulled the master off the bike and dismantled it. It was pretty foul. I found that the inlet hole below the elbow was gummed up so cleaned that up, and rebuilt the master using a kit.

This morning I tried to connect a syringe to the front caliper thinking bleeding the system would be a piece of cake. Once again, with all the lines connected to the m/c and the nipple open, I just couldn't push any fluid back to the reservoir. Tried pumping via the pedal but can't push fluid more than an inch or so past the nipple. Tried the same procedure with the rear caliper with the same result. The second pic shows how far I can get the fluid before it just won't go any further.

I have stripped and cleaned the rear caliper, but as I don't have a compressor, I only managed to pump one piston out. It looked incredibly clean and in good shape. I can't think that stuck pistons would make it impossible to push fluid through the system? I've pushed clean fluid thought the lines, up to the m/c so don't think there's blockage or excessive dirt in the lines...

Also, would a lack of fluid cause the brake light pressure switch to not operate?

Any suggestions appreciated....Thanks,
Old master cylinder Rear caliper
 
Yes, you need pressure to turn on switch. You may need to suck the fluid through the system with a pump hooked up to bleeders. I have one that works off my air compressor. You can get a hand pump one also.
 
Yes, you need pressure to turn on switch. You may need to suck the fluid through the system with a pump hooked up to bleeders. I have one that works off my air compressor. You can get a hand pump one also.
Will one of those hand pumps produce enough pressure? I've never had this much hassle bleeding brakes before!
 
That's what I used for 30yrs before I spent the $ on the mighty vac. Pump it up and open the bleeder, close bleeder before gauge bottoms out. You don't have to pump foot pedal. The newer brakes have even smaller holes so new tool helps.
I used to put caliper up from master w/line stretched out on a chair so air goes up, Brembo's are a pain sometimes.
 
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