I wish I could remember who posted this. I believe it came off of the Sprinter Forum. In any case, I too thought I had spongy brakes. I asked the dealer, and they checked out OK. However I was still skeptical. I did the 'check' listed below and was satisfied that my brakes were OK. The process references the 02-06 Sprinter, but it made sense to me for my 2012. The Sprinter brakes have a different feel than my car, but they definitely work. Note that I am not claiming that the 02-06 Sprinter braking systems are the same as later models. The procedure below just helped me think thru what was going on.
FWIW, I do a lot of mountain driving, and have never had any brake issues (other than the odd feel of them compared to a car).
"I receive a lot of mail and phone calls on the subject of ('02-'06) Sprinter brake pedal feel, especially a perceived "abnormal sinking pedal".
Many first time Sprinter owners or those who drive other vehicles (more often than their Sprinters) are alarmed by a brake pedal feel that they are convinced is abnormal.
Here's how to tell if your Sprinter brake pedal action is "normal" OR in need of immediate service attention:
Engine running, push lightly on the brake pedal. Mushy and sinking seemingly endlessly toward floor? This may be NORMAL.
Engage Park and handbrake. Shut off the engine and then step on brake pedal 4 or 5 times. This releases the vacuum from the booster servo and reservoir-you should hear a loud hiss when pressing pedal each time until the vacuum is expended.
Engine still OFF, ALL vacuum expended, now step on brake pedal and exert a steady force. Pedal should move a relatively short distance (compared to before) and stop hard without further sinking- REGARDLESS of how long you exert foot pressure. This is the TRUE test of the brake master cylinder. If pedal slowly sinks to floor during this test- you have a problem.
Now step on pedal again and exert steady pressure while starting engine. With engine now running- the pedal should begin sinking steadily toward floor. This is NORMAL and indicates the engine's vacuum pump and the brake servo are working properly.
Take foot off the brake pedal and run the engine for a few seconds at 1500
rpm to build vacuum. Go back to idle speed, step on the brake pedal slowly-see the difference and the much greater sink distance compared to engine "off" and vacuum released?
Still convinced it's excessive pedal travel? Try this test-Engine running, move Sprinter to a downward sloping driveway, place in neutral and SLOWLY creep down hill. Gently apply brake pedal pressure and note how little pedal travel is needed to stop and hold the vehicle-now push harder-see how much pedal travel remains?
This long, soft, pedal travel is a normal characteristic of the Sprinters vacuum booster design when engine is running."