If the master cylinder is engaged enough that it closes the hole from the reservoir, then heat from the brakes could cause the fluid to expand and cause what you're feeling. There should be enough free travel in the pedal that you see an initial spurt of brake fluid into the reservoir when the cap is off and the pedal pushed. A re-adjustment of the master cylinder pushrod should take care of that. If you continue to drive it that way, it'll wear the shoes prematurely


Kevin
1951 Chevy 3100 work truck
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Photos
1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car)
Busting rust since the mid-60's
If you're smart enough to take it apart, you darn well better be smart enough to put it back together.