I have a 98 4runner, Im having major brake issues, After my vehicle warms up and i drive for a bit my brake system builds and holds pressure like im holding the brake down. At first I thought it was my front calipers. Replaced both sides and no improvement. If i drive it more then a few miles i have to get out and bleed pressure in order to continue driving. Any ideas? or questions... Thanks

 

If the master cylinder piston can't return back out fully when the pedal is up, then fluid can't flow back into the reservoir. Then when it expands a little as it heats up, it puts pressure into the brakes with no way to relieve it.

Check that the brake pedal isn't being held down even slightly by something. Perhaps the brake light switch? If not that, the master cylinder might be sticky/gummed up and not wanting to return fully. There is a spring that pushes the piston back out, but it's not that strong.