Master Cylinder?
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Joined: Mar 2014
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From: Douglassville, PA
Master Cylinder?
My buddy has an '03 gasser and I was driving it yesterday 
The brake pedal was not firm like I'm used to and it dropped like it was losing fluid. A quick reset of the pedal slowed it's descent but did not stop it. The truck stopped what I would call mediocre but it did stop. Reservoir is full so I suspect the master cylinder is leaking past it's seals internally. As odd as this may sound I never had a failed master cylinder is all my life so this is a new one on me. Before just having him install a new one and hope that's it, does it sound like a master cylinder?
The brake pedal was not firm like I'm used to and it dropped like it was losing fluid. A quick reset of the pedal slowed it's descent but did not stop it. The truck stopped what I would call mediocre but it did stop. Reservoir is full so I suspect the master cylinder is leaking past it's seals internally. As odd as this may sound I never had a failed master cylinder is all my life so this is a new one on me. Before just having him install a new one and hope that's it, does it sound like a master cylinder?
Lead Driver
Joined: Jun 2013
Posts: 5,647
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From: north of Minneapolis, MN
It does. Look, at the end of the day, they are about $100, and it makes a great time to do a complete flush and fill of the brake fluid. Back i the day, you used to be able to purchase a rebuild kit for them. Don't know if you can any more.
Fleet Mechanic
Joined: Jul 2010
Posts: 1,753
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From: Los Angeles, Ca.
Sounds like a classic worn out master. A failed ABS module can also cause a sinking pedal, but the master is more common. I've done dozens of masters and had one bad ABS that caused it. If he wants tips on making the swap go easy PM me and I'll send you my phone number. I've posted the procedure I use but no clue how to find it and don't feel like typing that much.
Mountain Pass
Joined: Aug 2019
Posts: 177
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From: Abrams, Wisconsin
I agree with everyone above, but especially with flushing the system. All things break down over time and if he has a decent amount of miles on it think about how many heat cycles the fluid has been exposed too. At every oil change on my 2000 7.3 I open each bleeder and give it three four pumps at each corner and then top of the reservoir. This isnt as big of a job as flushing but I did that when I had to replace the rear calipers and figure this is a good way to keep cycling good fluid in.
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Joined: Mar 2014
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From: Douglassville, PA
Thanks for the responses. I would have just said to replace it but if he spent his $100 on it and it didn't solve it I'd have felt guilty for the bum steer. It turns out it was replaced by his shop not too long ago and they are going to replace it under warranty. If it sill acts up I'll point him towards his ABS system.
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