Cannot Find Location of Brake Fluid Loss

[Russell]

I always check fluid levels before moving the coach. On Friday, I found the front reservoir in the master cylinder to be only half full. It was full when I parked it three weeks earlier. I filled it with brake fluid, drove a few laps around the parking lot and the brakes felt fine, so I drove it home. At home, after 13 miles of driving and 11 traffic lights, the master cylinder was still completely full. After inspecting everything over and over I could not find a leak anywhere. At home, I started it, and pumped the brakes over and over trying to force fluid out so I could find the leak, still no luck, and no apparent loss of fluid in the master cylinder. I still have rear drums and front discs.

I think the master cylinder might be leaking into the booster. Is it conceivable that the vacuum booster might by "sucking" brake fluid out of the master cylinder while its in storage?

I think I need to replace my master cylinder and vacuum booster. Any thoughts; suggestions; tips; advice from those who have had a similar issue?

Historical reference: I installed all new brakes in 2016, wheel cylinders, calipers, pads, shoes, springs, brakes lines. Except I did NOT replace the master cylinder or booster at that time. My paperwork shows a new master cylinder in 2009. The booster is probably original.

Russell
'78 Eleganza II

 

[Christo]

I always check fluid levels before moving the coach. On Friday, I found the front reservoir in the master cylinder to be only half full. It was full when I parked it three weeks earlier. I filled it with brake fluid, drove a few laps around the parking lot and the brakes felt fine, so I drove it home. At home, after 13 miles of driving and 11 traffic lights, the master cylinder was still completely full. After inspecting everything over and over I could not find a leak anywhere. At home, I started it, and pumped the brakes over and over trying to force fluid out so I could find the leak, still no luck, and no apparent loss of fluid in the master cylinder. I still have rear drums and front discs.

I think the master cylinder might be leaking into the booster. Is it conceivable that the vacuum booster might by "sucking" brake fluid out of the master cylinder while its in storage?

I think I need to replace my master cylinder and vacuum booster. Any thoughts; suggestions; tips; advice from those who have had a similar issue?

Historical reference: I installed all new brakes in 2016, wheel cylinders, calipers, pads, shoes, springs, brakes lines. Except I did NOT replace the master cylinder or booster at that time. My paperwork shows a new master cylinder in 2009. The booster is probably original.

Russell
'78 Eleganza II

I was also thinking that it could be leaking into the booster, but that would empty the reservoir closest to the booster. I suppose it could leak into the booster then get sucked into the engine later when the engine is running. When you say "front" reservoir, can you clarify which one you mean -- closer to or further from the booster? Also, would you have been able to see if there was a wet spot on the ground, perhaps coming from an unexpected location?

I'm a big fan of Dave Lenzi's sensitized boosters which require less pedal effort. Applied also has them, but theirs use different internal parts that Dave says aren't optimal.

 

[Scotty]

in your search ,
don't overlook the proportioning valve?
you may also need to consider removing the rear wheels/and then drums to look for evidence of a leak.

I second Dave Lens's sensitized boosters, they also include an available secondary (aux) vacuum port for redundancy.

 

[Russell]

Thanks guys.

Christo, when I said the front reservoir, I mean the most forward reservoir in the master cylinder, (for the rear brakes) which would not be the one closest to the booster. I've got no visible wet spots on the ground, which is adding to my confusion. I was thinking that the brake fluid leaking into the booster was getting sucked in the engine and burned off.

Scotty, I looked at (with photos) the back of all the rear backing plates, near the bottom looking for brake fluid drips, as well the lower inside of the wheels and oily drips on the inside of the tires, with nothing found. But I think your right that I should pull the drums and have a closer look at the wheel cylinders. I replaced the proportioning / combination valve in 2018, (http://www.gmcmhphotos.com/photos/brakes/p64221-new-and-old-combination-valves.html), and did not see any fluid on it during my evaluation, but I will take another look.

I do have the sensitized booster and a master cylinder on hand that I already purchased from Applied a while back, but just haven't installed them yet. I think now is the time. But I will also pull the drums and have a closer look and the wheel cylinders. Since I'm there, I might as well repack the bearings.

Now I have a to do list for Memorial Day weekend.

 

[LQQKatJon]

Rear brakes will leak lots of fluid before you will notice anything…. You would have to pull drums off to see any evidence.

I drove well over a year adding fluid for the rear before each trip. Could be down 1” after a month of driving.

Just had some dirty brakes under the drum when I finally went through them. Never left any fluid puddle.

 

[Russell]

Thanks Jon. I'll pull the drums and check the wheel cylinders.