I recently replaced the front brakes and rotors on my Civic. Just prior to this, and now with the new brakes, I have experienced loss of brake pressure which requires me to pump the brake pedal to regain pressure and braking ability. This only has happened to me while on the expressway after traveling at a high rate of speed for awhile, never during local driving. I’ve seen a few things here about the master cylinder with the Hondas- wondering if my high speed brake loss means anything different? 122,000 miles on the car.
I would suggest that you drive VERY SLOWLY and VERY CAREFULLY to your mechanic’s shop for repairs. Your symptoms indicate an internal leak in your master cylinder, and total loss of brake pressure could take place without warning. Truthfully, I am shocked that you would continue to drive this car at high speed with this major safety issue.
The brake loss sounds like there is something causing the brake fluid to get too hot and boil when at high speed. This is common on smaller cars when they run a replacement exhaust too close to a brake line. But other components getting too hot will also cause this. drive this car very slowly to a good mechanic, and explain the problem.
The mechanic checked the air pressure in the brake line once I had the brakes replaced. I was surprised when a week later I took my car on a trip for work and on the way home found myself pumping the brakes as I was exiting the expressway.
Sometimes, the problem is caused by the rear drum brakes being really worn out. The pistons go to the end and the fluid leaks out. The master cylinder reservoir is shared and the fluid goes over the baffle and leaks down into the rear brake section. You may not have this problem but check for low fluid.