Master Cylinder Differences for M101 Surge Brakes

Valence

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I wanted to discuss a minor difference between a older style master cylinder (probably original) off my 1997 Pribbs M101A3 trailer.

First, a side-by-side comparison. The new one is (obviously) the shiny silver, which is the "marine" (wet environment) finish:
Part Number: 10614-M
https://www.crofttrailer.com/10614-m-hydro-act-actuator-master-cylinder-marine-finish/

2019-06-14 16.35.08.jpg 2019-06-14 16.35.28.jpg 2019-06-14 16.36.01.jpg

For reference, the original master cylinder used this vented cap with the 'U' shaped pipe and coolant hose:

2019-06-14 16.36.35.jpg 2019-06-14 16.45.47_zoom.jpg

As can be seen in the third picture, the new master cylinder has a wider opening, 1-5/8" to be exact, comes with a different square topped vented cap, and uses a rubber gasket instead of a copper crush ring. I do see that Croft also sells an alternate, tall, rounded top vented cap, for an additional (at the time of this writing) $60. What is the benefit, if any, of this cap?
Part Number:
850279
https://www.crofttrailer.com/850279-hydro-act-actuator-metal-vented-filler-cap/

850279_AV__16381.1376057881.jpg 850279_Ind__24609.1376057880.jpg part_breakdown.png
 
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Valence

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I think this is the master cylinder I used:
https://www.crofttrailer.com/10614-hydro-act-actuator-master-cylinder/

It's lower profile like the original.
Yessir, that's the exact same as what I have listed and linked, except without the overly expensive "marine" finish. Same part number, just without the "-M".

Good point about the standard cap on the new master cylinder being lower profile. I could see that optional rounded cap easily getting bent up. I do have some concerns about the standard square topped cap with open vent holes on the sides of the square being directly exposed to the elements and rain.

10614_Ind__41380.1353009472.jpg
 
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McGuyver

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It's been a while since I researched all this, but as I recall, there is another similar master cylinder with the same hold spacing that *fits* but the reservoir sticks up higher, and the pressure port sits lower. This necessitates grinding the back plate on the housing in order to give clearance to the port. Mine had this one on it when I got it used from someone, but I didn't like it, so I sourced the correct one from Croft trailer supply. When I saw your picture, I thought that was the one you had bought. Could they have sent you the wrong one?

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McGuyver

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This is the wrong one I replaced. I think it is a Tico


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McGuyver

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Ok, looks like I'm chasing the wrong issue here. Looks like you have the right master cylinder, and are looking for the other cap? I never got one of the vented type, and just stuck with what it came with from Croft.

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Valence

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It's been a while since I researched all this, but as I recall, there is another similar master cylinder with the same hold spacing that *fits* but the reservoir sticks up higher, and the pressure port sits lower. This necessitates grinding the back plate on the housing in order to give clearance to the port. Mine had this one on it when I got it used from someone, but I didn't like it, so I sourced the correct one from Croft trailer supply. When I saw your picture, I thought that was the one you had bought. Could they have sent you the wrong one?
Croft sent me exactly what I purchased, the exact same thing as you have, just with the marine grade finish (which is what I had ordered). It is a perfect fit.

I apologize for not being more clear, but the master cylinder itself is not my question at all. I am not surprised that the cap is different (and larger diameter) after 20+ years. My questions are:

1. What is the purpose or additional benefits, if any, of that rounded top vented cap (Part Number 850279)? Its threads are also 1-5/8" in diameter so it would NOT fit the old master cylinder and is clearly intended for the new master cylinder.
2. Does it provide better weather protection or something for the vent holes?
 
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McGuyver

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No, my mistake. I should have paid closer attention to what you were writing.

I think the cap with the tube on it probably keeps rain water out better. I pulled my low profile square top cap off recently and found traces of water in the reservoir. I suspect it got in there through the two small pin breather holes in the cap when it was raining a lot this past spring.

I have seen the "hat" style cap on some m101 trailers before, but they don't seem to hold up, and quite often get bashed in.

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Zed254

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I suspect the metal 'hat' reservoir cover provides a small sealed volume of compressible air to allow for movement of the incompressible brake fluid in the system. The compressible air in the 'hat' can compress and expand (small psi differences) without allowing outside air into the system. This reduces moisture contamination. Its location at the top of the brake system means no air will become trapped in a brake line. Kind of a SWAG but best explanation I can come up with.

My M116A3 has this kind of resevoir cap with a rubber o-ring seal. My M1101 has a much more simple plastic cap.
 
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Tinstar

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Zed254 is correct.

The surplus hat caps were cheap and plentiful on eBay two plus years ago
I’ve been using them on both my trailers with zero issues.
No moisture, No corrosion and brakes work perfect.

89A3F780-4491-4F02-96DC-6198A5421352.jpg13E84B80-04E7-44D5-A18F-6CE497BC86BB.jpg

Plus it looks better than a hose hanging down in my opinion.
 

Guyfang

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The cap cost $1.93 for the military. I can give you a list of alt. part numbers, and suppliers if you want one. It can NOT be a worse cap then the old U shaped one. I replaced about a million of them on gen set trailers. Its about the first part damaged on a trailer. There are 16 different suppliers.
 

Valence

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Zed254 is correct.

The surplus hat caps were cheap and plentiful on eBay two plus years ago
I’ve been using them on both my trailers with zero issues.
No moisture, No corrosion and brakes work perfect.

View attachment 768167View attachment 768168

Plus it looks better than a hose hanging down in my opinion.
Thanks for the NSN! There's still one on eBay that is 3 times cheaper than Croft. Do you have any idea if the thread on yours 1-5/8" in diameter? (I just don't want to buy an NOS "hat" cap that is made for the smaller diameter opening rather than the larger 1-5/8" mouth on my new master cylinder. I did also ask this question to the eBay seller).


The cap cost $1.93 for the military. I can give you a list of alt. part numbers, and suppliers if you want one. It can NOT be a worse cap then the old U shaped one. I replaced about a million of them on gen set trailers. Its about the first part damaged on a trailer. There are 16 different suppliers.
If the caps with the U-shaped hose are the same as my old/original master cylinder, their replacements won't fit my new master cylinder as my new master cylinder has a larger opening (1-5/8")
 
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Tinstar

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Standby
I will walk down and check the opening size

Edit;
The opening on my replacement cylinder, with a new hat cap ( 2+ years ago) is 1 5/8”

The hat cap should fit yours just fine.
 
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Tinstar

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7F651CDE-4C40-48A6-AD86-D0B5865198F0.jpg

Original cap was smashed when I picked it up at Red River Army Depot.
Still did not leak.

The hose vent ones usually break off, allowing water inside.
 

McGuyver

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I got one of the hat type caps on order today too. Thanks for posting that NSN.

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Valence

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Standby
I will walk down and check the opening size

Edit;
The opening on my replacement cylinder, with a new hat cap ( 2+ years ago) is 1 5/8”

The hat cap should fit yours just fine.
Thank you so much Tinstar for measuring that so promptly for me. That was very kind of you and I appreciate it.


Guyfang, thank you too for the offering of alternate part numbers and vendors, but I think I’d rather go by the eBay route as an assured purchase.
 

Valence

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I suspect the metal 'hat' reservoir cover provides a small sealed volume of compressible air to allow for movement of the incompressible brake fluid in the system. The compressible air in the 'hat' can compress and expand (small psi differences) without allowing outside air into the system. This reduces moisture contamination. Its location at the top of the brake system means no air will become trapped in a brake line. Kind of a SWAG but best explanation I can come up with.

My M116A3 has this kind of resevoir cap with a rubber o-ring seal. My M1101 has a much more simple plastic cap.
Thank you for that description too!
 
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