Brake pedal feel concerns

[abittenbinder]

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. Doktor A

 

[mobileoilchange]

not to be confused with a spongy brake pedal which means air in the lines,
air in the lines from fluid boil over which could happen from moisture in the brake system and when the brake fluid over heats the moisture boils off creating air pockets.

or from moisture getting into the brake fluid because brake fluid is hygroscopic (meaning it asorbs moisture). all mastercylinders need a vent hole so the system wont create a vacuum. moisture also gets into the brake fluid through the vent hole. you can also get a spongy pedal from worn pads, or when the fluid level drops below the min mark.

i wonder why mb didnt go with the hydroboost? hydrboost master cylinders put out more pressure, less effort, safer in the event of and engine stall while driving, compact, last longer, etc.

 

[abittenbinder]

not to be confused with a spongy brake pedal which means air in the lines.

That's the problem- the Sprinter does have a normal "spongy" brake pedal feel and it is NOT due to air in the lines. That's the confusion causing many owners concern.

The true test is to expel all booster vacuum with engine off and WITH NO VACUUM test the brake pedal for abnormal sink and/or feel. If it NOW sinks- you have a master cylinder problem, if it NOW feels "spongy"- you likely have air in the system, especially if the brake system been recently worked on.

Overheated, moisture contaminated boiling fluid is another matter altogether. Doktor A

 

[mobileoilchange]

i understand exactly what you mean and i was just making a point to what can cause a spongy pedal after doing the process you explained in your first post.

all brake fluid asorbs moisture, since the master cylinder is vented.