Rear wheel cylinder
 

Hello, my 2001 Durango has spongy brake pedal. I have bleed all 4 brake line (in sequence) more than 4 times until the fluid was clear clean. I'm not loosing any fluid in the master cylinder but I'm getting air into the rear lines hours after bleeding it, only the rear lines. HOWERVER!!!!! no fluid is leaking OUT of the wheel cylinders, but can air get in????? even if not leaking??? please help/advice

I'm thinking the rubber lines on the wheels and maybe to blame. An inside loop slight crack that once you pressurize the line seals then relaxes and leaks maybe. Get under there and check them out. There is also the brake proportioning valve under the driver’s feet.

Out of the manual:

SPONGY PEDAL
A spongy pedal is most often caused by air in the
system. However, thin brake drums or substandard
brake lines and hoses can also cause a spongy pedal.
The proper course of action is to bleed the system,
and replace thin drums and substandard quality
brake hoses if suspected.

The back drums don't have rubber lines. I would check the fittings where the hard lines connect to the wheel cylinders and the bleeder valves, and if those are tight, replace the wheel cylinders as a pair. I just did mine. The cylinders are about $10 a piece. You might want to pick up a pair of speed bleeders as well to make it a one person job. The New cylinders usually come with an OEM bleeder already.