My brake pedal started acting strange today. Before, when I would press it, the car would brake normally, and the pedal would stand where it is. Today when I press the pedal, it starts braking normally, but then gradually starts braking less... if I keep pressing in the pedal it would brake the same, but eventually I will hit the floor with the pedal and it would just not brake any harder any more. Before I could only press the pedal about 40% to the floor and couldn't move it any more no matter how hard I press. Now it simply sinks gradually. What could be the problem here?

The car is Honda Civic 91 1.5, no ABS/ALB.

You have a fluid leak. If the level isn't dropping in the reservoir, the master cylinder is leaking from one side to the other. You will have to replace it or rebuild it. Your manual should give instructions for rebuilding it. It should be cheaper than a rebuilt one. Take it off, disassemble it, and clean it up good. Use a silicone grease to lubricate the parts as you put it back. Don't let any conventional oil touch any of the rubber parts. It will swell and ruin them. If you need more help, let me know.

Thank you very much for your reply.

Brake fluid level hasn't changed in the reservoir. However, I have driven the car again a few hours after this had happened, and now it's working fine again, I cannot push it in more than 50%. Just like it used to be.

If it does happen again - I know what to look for in my manual :)

On a side note, I like how my Civic's brake pedal feels. You can control how much you want it to brake. Not like that Volkswagen Caddy... in that car the resistance of the brake pedal does not increase with the strength you're using to push it in. So my parents tell me I brake like a lunatic once I have to drive them around in the Caddy.

On a second side note: Congratulations to you, labman, on your 1000+ posts :) You're doing a good job :)