Checking for leaks in brake line.

My car seems to have a pretty severe brake fluid leak, I will dump a bottle of brake fluid in and a half hour later the reservoir is empty and I have no brakes!

Is there a cheap, readily available liquid I can put in the lines just to see where it’s leaking?

Yeah, it’s called brake fluid.

If you are losing that much, it should be obvious - just look for the damp spot under the car.

…or inside the car. The master cylinder could be leaking, which sometimes results in fluid in the driver’s footwell.

No definitely not inside the car

Unless you hit something and damaged the hard lines, the only place you are going to get a leak is at the master cylinder and at each brake.
(At least, 90% of the time).
Carefully check the hoses at each brake.

Also, brake boosters have been known to fail, and suck brake fluid into the engine, but that’s rare. You’ll see blue smoke if that’s happening.

Well it may be hard to find any fluid since he is MOVING when he uses the brake pedal.

How about STOP, and put the emergency/park brake on, and turn off the engine, and pump the pedal, and make it hard. Then when its really hard, hold that pressure on, like you were trying to stop the vehicle when the engine isin’t running. Does it drop away, or stay in place ? If it drops away… check for the leak ?

anyway, if you aren’t going to fix this problem yourself, which you aren’t if you don’t know what you are doing, then why not get a mechanic to diagnose the problem, whats the point of you diagnosing it ?? If one wheel’s part has got too old, then all four may be too old…
It seems to me that if you find its a bad slave cylinder, it may be time for 4 slave cylinders and master be overhauled-- replace the rubber seals that hold the pressure in.

In places not as dry as Arizona, sometimes brake lines rust through.

FYI, at the start of winter, I had the brake line break on my Avalanche. It was in for new brakes at the time. By the time he was done, everything on the brake system except the master cylinder and 2 calipers was replaced. Shade tree mechanic’s bill was still almost $1500.

I had 2 cars have rusted out brake lines (in the NE salt belt). One was under recall for that (Subaru) which they paid the tow and replaced the lines which from what I understand was something like 8-12 hours of labor to do. Mostly during this recall they just inspect and spray the lines at a critical spot with ‘rust inhibitor’ aka wax, unless there is serious rust which qualifies for full line replacements. So check for recalls, it doesn’t matter how old the car is, how many miles, from what I understand they have to do it.

But besides that yes if you keep adding eventually it will start dripping, however it may be leaking inside the car, even running down filling up the wheel well. If the pedal sinks when you step on it you may be able to hear it if it is quite enough. Even if you can’t hear it, just use the brake while still (you can run the engine to use the brake booster), and you will eventually form a puddle, or perhaps a lot of smoke if it drips into something hot enough.