Dry Master Cylinder - No Sign of Leak

[hrpearce]

My 70's C60 Chev custom cab was loosing break fluid with no obvious leaks for two years until I found it was leaking into the booster and burnt in the engine getting rid of the evidence.
Phil the cab on my blitz has no hole in the floor I have to get under the blitz with a mirror to check the fluid level.
I'm not thinking straight just realised fluid can't get into a blitz booster.

[Lynn Eades]

So are you guys implying the stuff just evaporates?
The reality is it has leaked past the cups and out of the boots. if it does it at the master cylinder, the leak should be able to be observed and also at the wheel cylinders it should be able to be seen at the bottom of the backing plates

In the end of the master cylinder (first bits in, rubber seal and a tin bit with a rubber seal in the middle) is a residual line pressure valve (drum brakes). This is there to keep a "column of fluid' in the lines so as to reduce the amount of pedal travel required. This also keeps a little pressure on the cups and keeps the lip of the cups sealed against the cylinder bore. If the vehicle sits for a while this pressure can "leak off" (back to the reservoir) This then lets the cup lips relax back from the bore which in turn can let fluid seep past the cups. (Think about the grooves carved in the bore by even the finest of hone stones- little tracks for fluid to seep from) When you use the vehicle again, a small amount of fluid will probably burn off with use and you may not notice that it had leaked (or the loss of efficiency) Likely not much worse than driving through water and having to dry your linings out.

In your case Phil, I would be surprised if you now find anything wrong.
Knowing that you do your best to keep things in good condition, your system is far more likely to leak at rest (no pedal pressure) than with 35 pounds applied to your pedal. If it is leaking anywhere else a careful look must pick it up.

As an example, if you have ever had a vehicle with a tandem system and if you have ever rested your foot on the pedal and it slowly sunk (internal leak)
Quite often if you push harder, it will hold (not always, and yes, it is time for repairs)

[rob love]

I was going to say the same thing: the fluid will be more likely to leak at rest than at pressure, as the tension is not on the cups.

The leaked fluid will dry eventually, but will usually leave some staining to let you know where it left the system. Question is, was your system full in the fall, or near empty to begin it's winter rest.

Not all brake systems have the residual check valve. I know I have to remove them on the M135 when I use a M35 master cylinder, otherwise the brakes drag for 2 or 3 seconds.

You can also have cup expanders in the wheel cylinders to help prevent the seepage. It uses the spring pressure in the wheel cylinder to force the lips of the cup outwards.

I have always said the sitting is worse for these vehicles than using them. In my location, the temperature over the year will vary by almost 80°C over the year (we see almost -40 in the winter and almost +40 in the summer). The miracle is that the systems work at all. The other miracle, when I think about it, is that anyone chooses to live here.

[Lynn Eades]

Good points Rob. Especially the last one
The general rule for residual line pressure valves is;
Drum brakes have them and disc brakes don't. (not directed at you Rob )

[Phil Waterman]

Hi Rob

As things happened the trucks just got driven into the shop after the Summer Rally and sat until the end of November before I could do anything on them. So no they didn't get a fluid check before going into storage this year.

Well the system in the HUP sat at pressure for 24 hours did leak down a little 41 to 30 lbs still no sign of any leakage. With out re-stroking peddle increased the peddle pressure up to 55 lbs and then left for an hour. There was no sign of the peddle having creeped down.

So tomorrow when the weather is better I'll take the HUP off the jack stands and take it for a drive and see how the brakes feel. Taking each of the trucks for a nice long drive is part of my spring ritual, got to burn up all that stale gas in the tank and then get fresh.

Cheers Phil