Leaking brake fluid - rubber lines ok- how to check piston seals

[SilverSoarer]

Brake fluid reservoir goes from max to min within 4 days.

Have all new genuine Toyota pads on all 4 corners along with new rotors.

Rubber brake lines and caliper nipple bolts do not appear to be leaking.

No obvious puddles at tire corners.

How can I verify that the pistons in the caliper are not leaking brake fluid - as all four of them seem dry - ?

[carsrus]

They usually dont leak externally. Look around all your lines carefully, 4 days means is a rather slow leak. Wear safety glasses and with the car off pump the pedal 5-6 times and then push down HARD. The goal is to rupture the line where its weakest.

[Nark]

My MB just recently has the same problem as your. Both front calipers are leaked. Look behind each tire and see as if animal piss on it. If not then your master cylinder has internal leak and it went inside booster pump. Keep in mind if one failed, you'll most likely need to change or repair all calipers and flush the entire system.

[JohnGD]

+1 may be an internal leak from the back of the master cylinder. Check the joint between the brake booster. But since there is a gasket there, it can leak into the booster. Try pulling the vacuum hose and check for fluid in it.

[SilverSoarer]

Found the slow leak.

It's on the hard brake line that runs down the firewall near the power steering rack - will upload a pic when weather is better.

Will try tightening fittings (if there are any loose ones) and try to isolate the leak to the individual line.