Fluid leak behind master cylinder

[Angelo]

I noticed either brake or power steering fluid all over my chassis and finally found the source of the drip. It was almost directly below master cylinder, and I found the source to be between the MC and whatever is behind it(brake booster?). Its a 1996 6.5l diesel k3500. Is there a seal between these 2 parts or does a fluid drip here mean that the MC can be replaced to remediate leak?

Glad I got the hard part done.

 

[GoToGuy]

A seal around the shaft that moves in and out as you apply brake pedal pressure. Most probable. You know the fluid stays within the brake master right? Any external leaks are a problem to be repaired.

 

[kolgeirr]

I noticed either brake or power steering fluid all over my chassis and finally found the source of the drip. It was almost directly below master cylinder, and I found the source to be between the MC and whatever is behind it(brake booster?). Its a 1996 6.5l diesel k3500. Is there a seal between these 2 parts or does a fluid drip here mean that the MC can be replaced to remediate leak?

Glad I got the hard part done.

That unit behind it is a hydroboost unit - a brake booster that uses hydraulic pressure from the power steering system for power. Common in turbodiesels as there's no engine vacuum under boost.

AFAIK, both of those parts have shaft seals there. You can leak brake fluid from the master cylinder, or power steering fluid from the hydroboost unit, at that same place depending on which seal(s) are toast. I don't think there's a gasket or seal between the two units, just the shaft seals, but I may be wrong - haven't had hands on a GM hydroboost yet, just Ford.

 

[HotWheelsBurban]

Power steering fluid feels a little more oily, more viscous than brake fluid. If the hydro boost unit is leaking (and the big seals on the mainshaft are commonly where they do) DO NOT drive the truck till it's fixed! The hydro boost unit went out like that on my crew cab, Rawhide, last year, and you don't realize how much force and effort are required to move one of these big boys until you try to turn and stop one at low speed! It took everything I had, to get the wheels turned with no power assist, and I had to do it in the middle of a very busy street in Houston, Texas. And my truck is a 350 so yours has a fair bit more weight on the front end, being a diesel.

 

[HotWheelsBurban]

Bad master cylinder usually --but not always -- the pedal will go almost all the way to the floor before it stops, and feels "spongy". Are you having that issue/symptom too, or is it just leaking?

 

[Pollo Dustino]

Your brake fluid looks quite low. Check your brakes, if you have lots of pad material you probably have a master cylinder leak.

If you have a lot of miles on the vehicle it may be worth it to do both the master cylinder and the hydroboost unit at the same time. Take a look underneath the dash where the brake pedal pushrod goes into the booster. If you see any fluid dripping down or excessive moisture then the booster is leaking as well.

I've seen those leaks on GMs, Fords, and Dodges because they all share similar booster designs.

 

[Angelo]

That unit behind it is a hydroboost unit - a brake booster that uses hydraulic pressure from the power steering system for power. Common in turbodiesels as there's no engine vacuum under boost.

AFAIK, both of those parts have shaft seals there. You can leak brake fluid from the master cylinder, or power steering fluid from the hydroboost unit, at that same place depending on which seal(s) are toast. I don't think there's a gasket or seal between the two units, just the shaft seals, but I may be wrong - haven't had hands on a GM hydroboost yet, just Ford.

Thanks. So it could be either or both

 

[Angelo]

Your brake fluid looks quite low. Check your brakes, if you have lots of pad material you probably have a master cylinder leak.

If you have a lot of miles on the vehicle it may be worth it to do both the master cylinder and the hydroboost unit at the same time. Take a look underneath the dash where the brake pedal pushrod goes into the booster. If you see any fluid dripping down or excessive moisture then the booster is leaking as well.

I've seen those leaks on GMs, Fords, and Dodges because they all share similar booster designs.

Thanks, so it could be both. I'm thinking to upgrade the master cylinder with the NBS style that is popular. The YT videos suggest part number NM4073 will fit the OBS to 1999, but they're doing it on gas engines only. It looks the same as the NBS master cylinder for 6.6L but I'm not sure if it's the same since my brake booster is different from gas. Maybe I'll do this upgrade and hopefully it fixes but if not, I get an upgrade anyways.

 

[Angelo]

I tried replacing the seals in my power brake booster but that made the leak worse. Ended up replacing the whole hydraulic power brake unit for $240 and leak is no more

 

[GoToGuy]

Without inspecting the seals, the seal areas , and some people are a little rough with parts. If the shafts have worn points, no sealing. Did you use the correct size and materials? Not everything is compatible. Brake fluid orings, don't play with mineral oil based orings. One little nick in an O ring and it's a " do over".