748 Stiff Brake Lever

Are the calipers sticking on?

 

Can you put the bike on an axle stand, preferably a front one? If it were me I'd check whether the same thing happens when the bike isn't being ridden. Just pump the brake lever a few times (maybe a lot of times) and see if it still seizes up. If it's on a front axle stand, when it's seized you can check if the front wheel still spins or not, this will determine if the calipers are freeing off or not.

If it doesn't happen when static but only when being ridden then it would seem to point to heat causing something in the caliper to expand and stick.

 

Have you done anything to the lever itself, could you have made any adjustment to the plunger.

 

Insufficient free play at the lever between the plunger pin and the piston in the master cylinder would be my bet.

 

Early non adjustable levers had a grub screw that activated the plunger. If it was screwed out it would reduce the gap between the lever and plunger which, once the fluid had warmed up a bit, would result in the brakes seizing/sticking.

 

You ask @Birdie or someone else who still owns a 748 or 916/996/998. I can’t remember the exact details any more. I sold my 748R in 2007.

 

Does your lever have this type of grub screw? If so, they are usually glued in position with araldite/resin.
If it’s loose, I suggest screwing it in a bit and going for a ride to see if the brakes stick again.

 

You could be warm with this post, there is a fluid return port/gallery in the master cylinder body between reservoir and cylinder bore. If this is even partially blocked the system starts to 'pump up' giving your exact symptoms. I can't remember bore size exactly but I think it's around 0.6 mm.

I totally agree R, this can cause the same result as above if stroke screw has moved because it's loose or has been adjusted incorrectly. Just like the rear system, the play could appear to be correct but might diminish to zero when brakes are hot and exacerbated if slave cylinders sticking slightly and/or used during hot weather.

 

Do you have adjustable levers? Poor picture (and example) but it’ll have a round knob you can adjust to bring the lever closer or further.
I know that’s the clutch side but the brske’s the same.

 

It's not what's used for free play adjustment, I was just determining what levers you have. When the round knob is adjusted it pivots around the red circled pin either reducing or increasing the distance (green) between the lever and the grip. The yellow circled pin is where the lever which actuates the brake cylinder piston rod is connected. I'll take a look at mine later and post more pics to try and help.

Check if the symptoms occur when the bike is static that way you can determine, and eliminate one or the other, whether it's purely brake fluid pressure causing the seize or heat build up. If the former I'd investigate Chris's suggestion.

 

Having checked mine (99 748) there’s no free play adjuster between the master cylinder piston and the lever; the lever rests against the piston that’s all.

When you sucked the old brake fluid out with the vacuum pump did you end up draining the reservoir, and thus possibly the piston, completely, or did you keep it topped up with new fluid?

 

Here is mine, they are after market levers but just the same as my standard ones, the adjuster is the screw for the flat head screwdriver, I have about 5 to 10mm play at the end of the lever before I feel any sort of pressure other than the lever return spring.

Steve