Brake pedal goes to floor "sometimes" only

Question

Driving a 2007 Volvo S40, automatic transmission.

As title states, my braking pedal sometimes goes to floor during driving, mainly when I'm in a traffic jam and using the brakes actively. It also happened a few times while driving on highway at higher speeds, so the first 40-50% of pedal travel it looks like it doesn't brake at all, then it brakes, but it's not even close to the "usual braking" feeling that happens when it's working as it should and at like 20% pedal travel I instantly feel like the car is actually going to stop.

Unusual things that happened these days was that my car got raised on a stand and the engine was running, so as the car was "floating" the ABS light turned on, but after driving on road for a couple seconds it turned off.

So the biggest issue is why does the pedal go to floor only sometimes (like if it was something horrible there, it would probably always go to floor).
This is probably a pretty stupid question, but it's my first car and I'd rather look stupid online than hit an utility pole.

Answer 1

It's not a stupid question at all! You are right to be concerned, brakes are critical for safety and you need to deal with this before you get in an accident. The ABS light turning on while it was on the lift is probably a red herring, especially as it turned off once it was back on the ground. An ABS problem isn't likely to cause inconsistent braking, what you are describing sounds like you have air in the lines, although it could be a problem with the master cylinder.

What you do about this depends on how handy you are with cars. Taking it to a mechanic is the course of action most would do, and what I would recommend unless you have experience (or want to gain experience) in car mechanics. If you want to do a few checks yourself then I'd check the level of the brake fluid reservoir first, if it's low then you probably have a leak as brake fluid doesn't just disappear. Next I'd inspect the master cylinder for leaks, then the brake lines as far as I can see them, all the way to the calipers if possible. If there's a leak it's probably going to be pretty obvious. If there's a leak then you fix the cause and bleed the system thoroughly.

If there isn't a leak then the problem is probably the master cylinder. The master cylinder has a bunch of seals in it, if one of them goes you can get inconsistent pedal travel and uneven braking. The solution there would be to replace the cylinder with a new or refurb unit, or refurbish it yourself with a kit.