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1Back to top Go down   Rear brake caliper pistons Empty Rear brake caliper pistons on Wed May 16, 2018 11:57 am

@kustom_moto

@kustom_moto
Silver member
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Guys hoping to find some help please. I am having trouble with the rear caliper pistons, with the brake bled and free from air when I push the pedal down the pistons move slightly but not enough to bite on the disc, when I release the lever they retract slightly.

Is it normal for them to retract slightly? Are the piston seals tapered?

    

2Back to top Go down   Rear brake caliper pistons Empty Re: Rear brake caliper pistons on Wed May 16, 2018 1:13 pm

92KK 84WW Olaf

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At the brake pedal you will see a rubber boot on the end of the master cylinder.

There is a movable receiver inside that boot for the end of the brake pedal pin. Check that it is properly seated, if not the pedal pushes it to one side and does not force the piston into the master cylinder to operate the brake. Also take the plastic cover off the caliper and check the pads move freely on the pins.

The pads and pistons do retract slightly when you release the pedal.


__________________________________________________
1992 K100LT 0193214 Bertha Alaska Blue 101,000 miles
1984 K100RT 0022575 Brutus Baja Red 578 bought 36,000 now 89,150 miles
1997 K1100LT 0188024 Wotan Mystic Red 689 58,645 now 105,200 miles
1983 K100RS 0011171 Fricka 606 29,495 miles Damn K Pox Its a Bat outta Hell Now 36,188  miles
    

3Back to top Go down   Rear brake caliper pistons Empty Re: Rear brake caliper pistons on Wed May 16, 2018 1:21 pm

Dai

Dai
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Check the rear caliper. If you have the original pins in the chances are that they're pretty rusty. If they are, show them to a rotary wire brush in a drill and also consider replacing them with stainless steel pins from Motorworks. Check the pads; they can also rust up and prevent free movement in the caliper. Finally, around that area, scrub all of the dirt out of the caliper as it can build up in the corners and prevent free movement. The only thing is it's a bitch to get off; you need to take the rear wheel off first.

If nothing obvious shows up around the caliper then the most likely source of the problem is the master cylinder. My bet is either that the return hole is blocked or the m/c seals are toast. The easy bit first; make sure the brake pedal has a very small amount free play in it and is not permanently pushing against the rubber. If it's hard up against the rubber block it can push the piston back just far enough to cover the return hole. DAMHIK.

To check the free play you'll need to peel the boot back carefully; if no hydraulic fluid runs out then it's not the seals.

If neither of the above works then there is probably crap in the return hole. The good news is that dismantling the rear m/c is stupidly easy (assuming nothing is corroded up). Dismount it from the footpeg hanger (yep: take the hydraulic hose off). Give it a good clean. On one side is a tiny allen bolt; this holds the piston, spring and seals in. Unscrew the allen bolt and carefully remove the piston assembly. If the seals are intact you don't need an overhaul kit. Clean everything up very carefully and reassemble it with silicon grease (dip it all in hydraulic fluid if you have no silicon grease).

Try bleeding it again.

Edit: as Olaf says about the rubber block. The metal cap on top can also fall off; I glued the my last one back on with Gorilla glue. Then I got really peeved with it (p!ssed off actually) and replaced it with one made from stainless. IMHO it gives a much better feedback from the caliper.


__________________________________________________
'83 K100 upgraded to K100RS spec, '87 K100RT
Others...
'78 Moto Guzzi 850-T3, '79 Moto Guzzi 850-T3 California,'93 Moto Guzzi 1100ie California
    

4Back to top Go down   Rear brake caliper pistons Empty Re: Rear brake caliper pistons on Wed May 16, 2018 2:34 pm

@kustom_moto

@kustom_moto
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Wow thanks guys for taking the time....gives me hope! I recon I have spent the best part of a day on this and was no further forward than when I started! I should have said at the start that all components are extremely clean,  the MC is actually polished. 

 MC and calipers have all been rebuilt although I have not used new piston seals (as they looked ok) and have not used a MC rebuild kit. The MC was leaking badly until I fitted an EPDM seal behind the top sweeper bushing, now it seems fine.

Now...the allen bolt which holds the MC piston in place the head was damaged so I replaced with a little M5 bolt the same length,  I am now wondering whether that could be making tje piston cover the little return hole? The original bolt has a thinner tip to it.....hmm?

Question, if that return hole is blocked (I did blow it out) would it effect the pressure it could achieve? I will double checknthe pads slide properly on the pins but they have again been wire wheeled and lubricated.

The issue really is lack of pressure. At no point today have I once felt a resistance in the footpedal or a pressure build up, although brake fluid is pumping......

    

5Back to top Go down   Rear brake caliper pistons Empty Re: Rear brake caliper pistons on Wed May 16, 2018 3:32 pm

Dai

Dai
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@@kustom_moto wrote:MC and calipers have all been rebuilt although I have not used new piston seals (as they looked ok) and have not used a MC rebuild kit. The MC was leaking badly until I fitted an EPDM seal behind the top sweeper bushing, now it seems fine.
This. I bet the fluid is going backwards and forwards around the damaged o-ring seals.


__________________________________________________
'83 K100 upgraded to K100RS spec, '87 K100RT
Others...
'78 Moto Guzzi 850-T3, '79 Moto Guzzi 850-T3 California,'93 Moto Guzzi 1100ie California
    

6Back to top Go down   Rear brake caliper pistons Empty Re: Rear brake caliper pistons on Wed May 16, 2018 4:05 pm

@kustom_moto

@kustom_moto
Silver member
Silver member
If the o rings in the MC were damaged would it not leak fluid?

    

7Back to top Go down   Rear brake caliper pistons Empty Re: Rear brake caliper pistons on Wed May 16, 2018 6:02 pm

Dai

Dai
Life time member
Life time member
If just the front one (the one closest to the hose) is no longer sealing you wouldn't get a leak; you just wouldn't get any pressure. BTDT with a Moto Guzzi rear caliper.


__________________________________________________
'83 K100 upgraded to K100RS spec, '87 K100RT
Others...
'78 Moto Guzzi 850-T3, '79 Moto Guzzi 850-T3 California,'93 Moto Guzzi 1100ie California
    

8Back to top Go down   Rear brake caliper pistons Empty Re: Rear brake caliper pistons on Thu May 17, 2018 3:36 am

@kustom_moto

@kustom_moto
Silver member
Silver member
I recon you are bang on actually. Ill be taking the MC apart again later on to take another look......will let know what I find.....

    

9Back to top Go down   Rear brake caliper pistons Empty Re: Rear brake caliper pistons on Thu May 17, 2018 10:16 am

Point-Seven-five

Point-Seven-five
Life time member
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I had the same problem with the master cylinder on my K75RT.  No amount of rebuilding, cleaning, etc. provided any joy.  Trying to make it work was a total waste of time.   I wound up binning it and retrofitting an $11 Chinese master cylinder I got on eBay.  That master cylinder has worked flawlessly now for a full riding season and over 5,000 miles.

http://www.motobrick.com/index.php/topic,9364.25.html

My experience begins at reply #29.


__________________________________________________
Present:
1994 K75RT
1991 K100RS
1988 K100RS SE

Past:
1994 BMW K75S
1992 BMW K100RS
1982 Honda FT500
1979 Honda XR185
1977 Honda XL125
1974 Honda XL125
1972 OSSA Pioneer 250
1968 Kawasaki 175
    

10Back to top Go down   Rear brake caliper pistons Empty Re: Rear brake caliper pistons on Thu May 17, 2018 2:50 pm

@kustom_moto

@kustom_moto
Silver member
Silver member
This Chinese MC, is it the one that has links to it at the very start of the thread?

Also, I hear the MC from the K1100 also fits. Will I have the same issue eith this version do you think?

    

11Back to top Go down   Rear brake caliper pistons Empty Re: Rear brake caliper pistons on Thu May 17, 2018 3:15 pm

Dai

Dai
Life time member
Life time member
Before you start with a complete panic Very Happy , strip the m/c and look up inside the body. If there's any sign of the anodising being polished off (shiny silver bits showing through) then it's unlikely to take another overhaul. If it's still all black in there (albeit smoothed out where the o-rings rub) it should take another overhaul.


__________________________________________________
'83 K100 upgraded to K100RS spec, '87 K100RT
Others...
'78 Moto Guzzi 850-T3, '79 Moto Guzzi 850-T3 California,'93 Moto Guzzi 1100ie California
    

12Back to top Go down   Rear brake caliper pistons Empty Re: Rear brake caliper pistons on Thu May 17, 2018 3:31 pm

@kustom_moto

@kustom_moto
Silver member
Silver member
Problem is is that the overhall kit from Motorworks is £60!!! 

I am still unsure exactly why the piston will not pressurise, the rubber pressure seal looks to be it excellent condition. When I push in the piston and keep my finger over the end hardly any air feels like it wants to escape then when the piston returns I csn feel a definite vacuum pulling my finger into the chamber slightly.

What is the exact issue here? Also, I don't believe this MC chamber was ever black anodised!

    

13Back to top Go down   Rear brake caliper pistons Empty Re: Rear brake caliper pistons on Fri May 18, 2018 8:10 am

@kustom_moto

@kustom_moto
Silver member
Silver member
I have now ordered the Chinese MC, hopefully this will provide more fruitful!

    

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