Guys, since writing the above I have replaced all the wheel cylinders and the 3 rubber final connection hoses to the brake backing plates, and bled the fluid again. Several test runs in hot weather, so far no lockup. Fingers crossed!

Thanks guys, will definitely followup on all your ideas. Very appreciated! It's just a mystery to me.

If you look at the master cylinder in the shop mNUAL, there is a small hole at the bottom of the fluid reservoir - this hole allows pressure in the system to bleed fluid back in to the reservoir as the brake fluid heats up.

This hole is just about in the middle of the reservoir.

If the master cylinder has been changed or worked on, this hole should be checked by putting something (like a paper clip) in the hole to see how deep it goes.

If it only goes in a slight amount - like 1/8" or so, the piston in the master cylinder is in the incorrect position, blocking the bleed hole.

You may have to empty the reservoir to see this hole.

As the other poster mentioned, the push rod may have to be adjusted to correct this problem.

BTW - some sort of bleed is present in most older master cylinders - My 54 Chevy had the same problem after a mechanic changed my master cylinder - in the case of my car, a special tool had to be used to set it correctly.

If you look at the master cylinder in the shop mNUAL, there is a small hole at the bottom of the fluid reservoir - this hole allows pressure in the system to bleed fluid back in to the reservoir as the brake fluid heats up.

This hole is just about in the middle of the reservoir.

If the master cylinder has been changed or worked on, this hole should be checked by putting something (like a paper clip) in the hole to see how deep it goes.

If it only goes in a slight amount - like 1/8" or so, the piston in the master cylinder is in the incorrect position, blocking the bleed hole.

You may have to empty the reservoir to see this hole.

As the other poster mentioned, the push rod may have to be adjusted to correct this problem.

BTW - some sort of bleed is present in most older master cylinders - My 54 Chevy had the same problem after a mechanic changed my master cylinder - in the case of my car, a special tool had to be used to set it correctly.

Possible clogged vent on B16 brake master cap or on remote brake fluid reservoir cap if you've updated to the B18 style.

Or faulty check valve in brake master.
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Eric
Hi Performance Automotive Service (formerly OVO or Old Volvos Only)
Torrance, CA 90502
hiperformanceautoservice.com or oldvolvosonly.com

The adjustment nut(s) on the MC pushrod might be adjusted tightly enough that the system doesn't allow for thermal expansion.

The shoe adjusters might also be adjusted with less 'slack' than they need.

If there's a chance that it's not related to heat but to use, the insides of the flex lines - one to each front wheel, one to the rear axle - may be swollen internally and preventing fluid from flowing away from the wheel(s) when the brake pedal is released.

Cam