R80ST with R100GS front forks and R1150GS calliper, loss of front brake pressure especially on severe left lean

[sidkn33]

Hello forum.
I have a non-standard but generally very effective front end on my 1983 R80ST = R100GS forks, original 19" ST wheel on a custom axle, with custom brake rotor and R1150GS 4 pot calliper.
Generally, the front brake works very well if a little soft at the lever, similar to way my 1992 R80GSPD: It's not a "modern" brake setup so I'm not expecting miracles.

Despite having pressure fed with new brake fluid, twice, I have a fairly severe pressure drop at the lever when the bike is leaned over hard, in particular when leaning left with the handlebars full lock to the left - it happens on the opposite side, but on the left I can almost pull the lever to the bar.
When the bike is upright the brake engages with much less travel on the lever.

It feels like air in the system, but that's mysterious given the use of a pressure feed and no obvious sign of air bubbles from the calliper when bleeding.
In general it doesn't affect braking because I'm not leaning hard over, far from it, I only really know about the issue because my garage is at the bottom of a slope and I have to do a full lock left hand turn to swing the bike around and back it into the garage.
Any suggestions?

 

[Bubb]

Pull lever back to bar, and fasten it there overnight…should force any air in the system up out of the system.

 

[sidkn33]

Pull lever back to bar, and fasten it there overnight…should force any air in the system up out of the system.

Thanks Bubb, I tried that - probably three times now - I think it may have helped temporarily, i.e. the brakes were initially firmer as a result, but the lever still moved more when the bike was leaned over.

 

[sidkn33]

Could I have some sort of air pocket or bad seals in the calliper? Or a dodgy master cylinder?
I'm trying to avoid shelling out £150 or so on a master cylinder if that's not in fact, the issue.

 

[Bubb]

Try bleeding it upwards…. ie. Use a syringe to force the fluid up from the caliper upwards..have the top off the master cylinder, and watch you dont overfill.
I use the syringe method when refilling brake fluid

 

[sidkn33]

Try bleeding it upwards…. ie. Use a syringe to force the fluid up from the caliper upwards..have the top off the master cylinder, and watch you dont overfill.
I use the syringe method when refilling brake fluid

Thanks, I'll give it another go - it's been a while since I last attempted to resolve, but I think I did exactly what you suggest, at least once previously.

 

[mikeyboy]

Maybe a curveball,
Loose wheel bearings?
Pushing the pad/ piston back.
Worth a looksee.

 

[sidkn33]

Maybe a curveball,
Loose wheel bearings?
Pushing the pad/ piston back.
Worth a looksee.

Definitely worth a look and not something I would have considered - despite having replaced the front wheel bearings very recently.
Going off thread @mikeyboy I did not (as I couldn't with my amateur skillset and lack of appropriate tools) pre-load the bearings properly - is that something that's 100% going to cause a premature bearing failure?
Thanks for your input.

 

[mikeyboy]

I’d say if the bearings have excessive pre load they almost certainly would fail prematurely.
If they have excess float ,on a tight lock the wheel and disc will rock,given that the caliper is fixed ,and the disc may be fixed or only semi floating,it won’t take much to move two of the pistons.
This won’t be helped much if you still have the 12 mm master cylinder which if this is the case,may be a bit small for the 4 pot in any case.
I’d have thought a 14mm would be better suited.

 

[sidkn33]

Thanks for the helpful input - much appreciated.
Interesting point re full lock - if there's lateral movement of the wheel between the forks from float in the (possibly under-preloaded) bearings it could pushing the brake pads back into the calliper - if I've understood you correctly?

 

[Gelandestrasse]

One other combination is full lock, lean over, then apply brake and cable tie, leave overnight.

 

[sidkn33]

One other combination is full lock, lean over, then apply brake and cable tie, leave overnight.

That's a nice and easy one to try.
Cheers

 

[Bubb]

Did you get it sorted?

 

[sidkn33]

Not completely, yet @Bubb
I'm currently rebuilding my ST cockpit - fitting a new Acewell speedo/tacho etc, which required disconnection of the brake hose to re-route it.
What I have done is fit new EBC sintered pads ahead of doing another complete bleed of the brakes - the EBC pads already made a big difference to stopping power, but can't comment more yet about the pressure-drop when on full lock and lean.

 

[Bubb]

Good luck with it, hope you get it sorted

 

[sidkn33]

I've re-bled and left overnight with the brake lever pulled tight.
The lever pressure drop is much less, but still happens on lean - I suspect a master cylinder issue, but as the brakes work so well now (with the new EBC pads) I'm going to leave well enough alone.