hydrooost master cylinder woes

[Tiger Joe]

79 c30 with a hydro boost.

short version- master cylinder will not stop leaking!

long version- had what we thought was a bad master, so we replaced with a rebuilt unit from carquest. put it on, it works well, BUT it will not stop leaking from the lid!

This is the standard metal aster cylinder with the metal lid, with the metal retaining clip you pop over it to hold it tight. the clip is very tight on this new unit.

initially we thought we were putting too much fluid in it. so we siphoned some out. Still after using the truck you can see wet streaks down the sides of the master cylinder where it is running out the lid.

Is there a solution out there for this? can we swap to a different style M/C? maybe a later plastic one with the plastic snap lid, or even aftermarket?

[Tiger Joe]

This is what the stock m/c looks like:

http://www.summitracing.com/parts/aa...30?prefilter=1

[walkerb]

Man I'm having the same issue. I've thrown a ton of money at it. I'm now going to research the hydro itself as the cause.
Frustrating to say the least. Brake fluid over everything

[mongocanfly]

If the hydraboost is the leak it would be power steering fluid leaking,,, not brake fluid..they work as 2 seperate units that are connected together.

[Tiger Joe]

yeah mine is def the master cylinder lid leaking.

Heck I even tried rebuilding the hydroboost just to rule it out.

I should have snapped a pic but you can clearly see streaks of fluid running down from my lid on my master.

I've thought about trying 2 things:
1- hitting the top of the M/C with a file to make sure its flat (maybe bad casting?)
2- if that doesn't work, using some silicone to glue the lid down.

[mongocanfly]

The top of the master clyinder should be a ground flat surface.sounds like it has to much fluid..is the rubber that's attached to the lid in good shape? Strange...

[Tiger Joe]

Originally Posted by mongocanfly

The top of the master clyinder should be a ground flat surface.sounds like it has to much fluid..is the rubber that's attached to the lid in good shape? Strange...

yes the whole m/c assembly is new in February or March. Only has probably 300 miles on it since then. Rubber on the underside of lid looks good to me, and like I said the metal retaining clip is really tight.

What should the fluid level be at? like how far down from the top should we fill it? ALL the brakes are new on the truck.

[mongocanfly]

The manual says 1/4" below the fill openening...mine is about 1/2" below it

[walkerb]

Fro the master cylinder I replaced the new one that the po had installed. New, not rebuilt. That solved the child out the top. I'm leaking fuild out the bottom of the hydroboost where the mc and hydroboost connect.

[Tiger Joe]

Checked it last night. It was definitely leaking out the lid just like I described.

I took a file to the lid, made sure it was FLAT. soon as I hit it with a file I could see how rough the casting was. I also used some clear silicone to glue the lid down. figure this might help me take up any excess voids.

set fluid level about 3/8" below the lid. letting everything sit for a few days to dry

[walkerb]

Keep us updated.
Was there any issue with the actual fuction of the brakes? Stop they are suppose to?

[80Sierra3500]

Interesting. I've been having the same issue with 3 different master cylinders through the life of the truck. I'm kinda wondering if there's water or air in the fluid down by the calipers. When it gets hot, it expands and raised the fluid level to where it leaks...?

[Tiger Joe]

Originally Posted by walkerb

Keep us updated.
Was there any issue with the actual fuction of the brakes? Stop they are suppose to?

There is no issue with the functioning of the brakes on the truck.

Back in feb-March my the truck needed rear brakes due to a leaking axle seal, so my dad decided to replace everything. All brakes front and rear are new, new M/C, and I put a new seal in the hydro boost. We replaced the m/c because the old one was leaking out the lid- which we thought maybe it was just old.

I doubt we have any issues with contaminated brake fluid. we bled the brakes a lot with doing all 4 wheels, plus bench bled the new master, never saw any issues with the fluid coming out.


We will keep an eye on the m/c and i'll update. so far I only drove the truck like 5 miles.

[Tiger Joe]

Well I just wanted to update for anyone else having the issues like I was.

Its been almost a month, and while we certainly don't drive the truck everyday, it has been used. After filing the lid and putting some silicone on it, I now have a leak free master cylinder.

[monte0185]

I was going to say swap it to a newer style from like a 98 with a 6.5TD. its a direct swap.