Dry Master Cylinder - No Sign of Leak

[Phil Waterman]

Hi All

Well spring has finally come to New England, so it is time to check and service the fleet. As CMPs have a single braking system be sure the master cylinder is topped up and the brakes are all working is a priority. C60S no problems only problem was one turn signal that didn't blink clean and replace bulb.

Next up was the HUP, which as you may remember had a full rebuild 4 years ago. Brake peddle went down about half stroke which was not normal as this truck has always had one inch of travel when the brakes are properly adjusted.

Removed the large brake access panel, yes late CMPs have a large removable floor section. Low and behold reservoir was basically dry.


Then rolled around on the creeper and could find NO sign of a leak. Refilled the Reservoir pumped the peddle slowly and the peddle level came back up to normal and is nice and hard. Applied the brakes as hard as I could and rechecked, again nothing. Left the HUP sitting on clean dry floor for 24 hours and no drips or drabs. Next step was to let it sit at pressure for couple of hours.

To do this position the porta-power on the peddle and brought the push up to 30lbs. For years I had tried to figure out how to apply 25-35lbs. to the peddle while turning the lower adjustment lugs, as is called for in the Manual. Well it turns out the solution to this is amazingly easy stand a bathroom scale on edge and use the porta-power ram to apply 25-35lbs.




Two hours into the test no pressure drop and no sign of a leak. So I left it for the day and will see what has happened in 24hrs.

Any bets on where or if I'll find a leak?

Cheers Phil

[Bob Carriere]

All the M 37 Dodge I had usually lost most of the brake fluid over the Winter months..... every spring we filled and bleed the system..... never did find the leaks.

Just did my C15a....... flushed all the lines with fresh military surplus synthetic....... seesm the old stuff looses it purple color so it makes it easy to see the fresh fluid going out...... we used and extended reservoir with a tube throught the MC fill cap and used a manual suction device to pull out the old fluid...... than jacked all 4 wheels and adjusted the cams...... brake real good, hi and tight. The fluid had been in since the system was rebuilt over 5 years and due for a proper purging. Never had a leak....even after months in the sea container rarely needed to pump more than once.

Cheers

[Clint Tauber]

My M-37 also consistently lost all its fluid over the winter, as does my M-38A1. Never found a leak on either of them, total mystery to me where the fluid goes. Just something I have come to expect. Dad's M-135, on the other hand, never seemed to go down.

[Phil Waterman]

Hi Bob

I've never had this leak down without a visible leak before. I've had the brakes loose a little over winter particularly if they are in the cold storage bay.

The remote reservoir seems to have been a common addition to Dodge MVs your comment was the second one I've seen today mentioning that.

My Pat 12 and early Pat 13 have ridiculously small floor holes to check the brake fluid.

The drop in the HUP though really caught me by surprise wonder if I just didn't drive it enough last year. My driving season got cut short in July at the Weare Rally when a hand problem went chronic requiring surgery so all three truck went it to winter storage in the first week of August.

Cheers Phil

[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

[Private_collector]

Originally Posted by Phil Waterman

The remote reservoir seems to have been a common addition to Dodge MVs your comment was the second one I've seen today mentioning that.

Previous owner of my grey CMP adapted the remote reservoir from a power steering unit, and it is installed behind drivers seat. Super easy to get at, as is a second one which is for hydraulic clutch.

[Frank v R]

Hi Phil , do you have a booster on your Hup?

[rob love]

Pull a wheel and drum, pull back the cylinder boot, and see what you find. If you find a bunch of white sludge, then you know you have a slow leak and it has dried out in the boots.

Since you are holding pressure, and are not showing visible signs of the leakage, it sounds like you have a few years to go yet. The remote reservoir, while not original, is a good indicator as to the serviceability of your system and it's rate of loss. I am installing them on several of the museum M-series vehicles for safety reasons.

[r.morrison]

Phil .....I too went through the same problem and with the same dilemma and it's nice to get everyone's 2 cents on "what makes things tick". HOWEVER.....he said...."tongue in cheek" .....Lynn's description, on my first reading was like listening to your high school gym class/health class teacher describe the pros and cons of sexual intercourse and good/aged condoms. Just kidding Lynn....my warped sense of humour. Apologies to all and Lynn....Robert

[Lynn Eades]

Phil, my immediate thought on your last post was" was it the porta-power that backed off?"

[rob love]

As an example of how little a loss of fluid will lead to a loss of pressure, the DND sent a message warning that the mico brake lock on the deuce was not to be used as a parking brake for anything other than very short term. The mico brake was a in-line solenoid that would lock the brake pressure into the system to be used for things like winching.

Once applied, the loss of as little as 3 drops would lead to the brakes releasing.