Today my brake pedal suddenly became too stiff with no free play. I can feel that the car is not rolling freely, in fact when I jack it up I find that all the wheels have significant resistance to turning by hand. There was no warning or special event, this just suddenly started. I don't want to drive the car until this is sorted out because I don't want to overheat the brakes.

My beetle has four after-market wide-five disk brakes from the wonderful John Sherman at Volks Conversions, with a dual-circuit master cylinder. I haven't adjusted/bled/touched anything for about a year so there is no recent messing about which could have caused the problem.

Any ideas what could be causing this?

Crack open a bleed nipple with bleed pipe attached and see if you can pump the pedal?

If pedal depresses and returns properly then MC may not be the problem. I mean if it was one of these MCs with residual pressure valves in from drum brakes then I'd say you'd have had problems fairly quickly with your brakes.

However, to have all four wheels hard to turn the MC would be the likely culprit.

If MC seems to work properly then bleed the brakes and check all wheels turn freely. Take a gentle drive to see if the same happens again.

If the MC doesn't work properly when testing or the problem recurs then I'd say it's the MC. Any single sticking caliper piston wouldn't cause all the wheels to bind.

Symptoms like no free play in the pedal

Check there is not something stopping the brake pedal returning all the way back. A pebble/rock under the tab that sets the pedals height could be a culprit

Back in the good old days we used to off road race a Baja with a standard master cylinder. Had problems with the brake pedal bearings corroding and seizing, the push rod to pedal corroding too, the pistons seizing and not coming all the way back, stones under the pedal return stop, and the pedals bending.
But the thing that sounds the same as your problem was interesting and hard to diagnose. We used to get dirt in the cylinder, through the push rod boot. The dirt built up between the piston and the circlip it comes back against, causing the piston to not quite come back past the hole that exposes the pressure side back to the reservoirs, which relieves the pressure. It only takes a millimetre and the pressure can’t be released, the brakes heat normally, pressure builds, and they start to lock on. Obviously it only gets worse as the temperature rises, until you have to stop and release a bleeder nipple. That of course only fixes it for a few minutes and it starts again. You have to take the cylinder out, remove the circlip and clean the outer end of the cylinder.
If that’s your problem the fix is easy. You won’t have anything like the dirt on the floor that we had, so it should be the fix for years. Perhaps just put a new cylinder in there and be done with all the problems for a very long time.
Good luck
Tony

Did you change the rubber brake flex hoses or are they still the old ones

if they are old then they break down from the inside out and cause blockages

I replaced the entire brake system about five or six years ago, new disks and callipers, new dual circuit master cylinder, new dual reservoir, and yes of course new flexible hoses as well.

I've vacuumed the entire area and managed to turn the pushrod a little bit, it seems to have helped a bit, I'll take it for a gentle test drive tomorrow. Bed time for me now.

Thanks for your help

I was waggling the pedal while I was vacuuming.

It is slightly better now but not quite as good as it should be. I'll have another crack at it on the weekend, and maybe pull the master cylinder too.

Check the push rod adjustment.