Brake pedal to floor intermittently
The last couple of days, the brake pedal has been normal on stops after start up. After a little while of stop and go driving, the pedal begins going almost to the floor. Still finally get to brakes, but the pedal is almost to the floor. Then, randomly, it returns to normal. Brakes catch at the normal position. I'm thinking master cylinder, but has anyone had this happen and what was the prob? It's full of brake fluid. 99 Laredo, 4.7 if it matters. Thanks in advance for any help.
2" BB front, 1.25" wheel spacers