NH TN65 Brakes Malfunction, Why?

[Reddirttractor]

Yesterday I was bush hogging a hilly pasture and had a brake system failure… luckily I had just reached the bottom of a hill when the anomaly happened! I’m trying to wrap my head around what happened, how to prevent it, and any advice or insight the forum may have to offer. some things about the situation make sense, others have me scratching my head.
As mentioned I was bush hogging in a hilly, narrow pasture yesterday afternoon. Machine had been running for about 4 hours and it was about 90 degrees outside. I had the brakes unlocked and was brake steering to make tight tuns in flat areas of the field.
One section of the field is fairly steep maybe 20%+ degree grade, not sure, but steep enough that you’d never want to side hill it.
On my final pass, I was coming down hill, and had geared down to A3. As I reached the bottom of the hill I depressed the clutch and both brakes to come to a complete stop.
Breaks felt soft, so out of instinct I pumped them a time or two, and then both sides, especially the left went to the boards. Absolutely nothing. I shut off the machine, put my parking brake on…which didn’t do anything obviously other than turn off the PB alarm. I thought I had blown a line or seal or something. During visual inspection the only thing I could find was that the brake fluid reservoir was about one finger below the max fill line. And that the float sensor had come loose from the cap. I’ll also add that I never had any lights on the consuls turn on or flicker during operation.
I hiked back to the shop to grab some NH brake fluid (NOT DOT) and give the machine time to cool… I was thinking about going through the bleed out process but decided to wait till the following day.
Next morning I head out, and both brakes were soft but functional again. I topped off the reservoir, reattached the float and pumped the brakes. Left side is solid, right side is softer but functional. Locked together they seem solid.
My theory is that my reservoir was low. And that when I came down the hill the left master cylinder sucked air. And then the equalizer line allowed air to get to the right cylinder. As the tractor sat over night the system self -bled the majority of the air out of the system. I never got a light because the sensor was detached(or I have a light out).

How can I avoid this? I suppose I should do a thorough walk around before each use, but come on, its a tractor, not an airplane. I check things every time I fuel up generally speaking.
Is there something else that could potentially be wrong?
I’m open to any and all suggestions, never want to be in that situation again. Things appear right, but both cylinders aren’t equally stiff. I suppose I’ll bleed the system.
Is it plausible that the system burped itself overnight?
Can a couple oz of fluid really put you in such a dangerous situation, is there no redundancy built into the system, or is my life in the hands of a plastic 9 oz reservoir?
What am I missing?
Tractors been running like a Swiss watch until this incident with the brakes. Part of me wants to take it to the shop, but if the issue won’t replicate, what’s the mechanic going to do that I cannot do myself?
Thanks for any advice or help. I’m curious if anyone else has experienced this.

 

[Harry in Ky]

If you look at the fluid reservoir you see the lines to the pedal cylinders come right out of the very bottom of the tank. It needs to be near totally empty before any air should get into that line. If the fluid level was above the minimum line at the time then it should have been safe to operate. One question might be did the loose float have any bearing on the situation. The filler, cap and float assembly are on the left side of the tank (almost directly above the outlet for the left pedal. I've found the floats off the stem before and never heard of any problems with brakes because of it. Only that the low fluid light won't come on. I'm not sure this is necessarily one of those "it's bound to repeat sooner or later" type problems. With the float fixed and level right my guess it should be good to go. I don't doubt you'll be rather cautious with it for a while, but then who wouldn't?

 

[Reddirttractor]

You bring up a good point about the Min fluid level, and I agree. One thing that came to mind after reading your reply. even though the float mechanism shouldn’t interfere with functionality, i believe it did interfere with my ability to judge the level of the reservoir. Now that I’m out in the shop inspecting at, the entire assembly other than the cap and plunger were in the reservoir…. had to fish it by hand with the aid of my Leatherman tool. Yesterday in the field, When I shined my lamp to illuminate the reservoir and see the fluid level, it may have been displaced and appeared higher …. Or I was seeing the top of the assembly and not the fluid from the angle I was at. I was either under the min mark or close too it. Low level plus the steep grade and sloshing… one of the inlets must have took a gulp of air.
I’ve gone ahead and topped everything off, and decided to shop for a new cap, sensor, and reservoir bushings/inlets… mine seem a little dry rotted… and I don’t think the sensor is supposed to fall into the reservoir. maybe I can find a deal on the entire assembly. Also I’m going to change up how I visually inspect on a walk around. Suggestions welcome are there any tips for topside visual inspection without pulling the float?

One thing that perplexes me is how the system self-bled overnight?
I feel like I’m just going to keep a close eye on things for the next couple uses until I get parts. However both sides aren’t feeling equal, the right side is squishy, but functional. I was planning to keep the pedals locked and hope that any residual air will work itself out and master cylinders will equalize.
Is this foolhardy or Should I properly bleed the system?
Should I even order parts?
Brakes are probably the most important part I’ve got with my terrain… don’t want to neglect, but also not throw good money at bad.
I wonder how I lost fluid, or how long the float has been detached?
I’m thankful this situation wasn’t a catastrophe.

 

[Harry in Ky]

I would say try bleeding that soft side. That might make difference. As far as keeping the pedals locked, I can't see where that will help anything other than make you feel better. Only the tighter side will be actually doing the braking.