Just went through this.
By-the-way your brake system on the 337 uses 5606 hydraulic fluid not brake fluid of an automobile. Automobile brake fluid is more "hydroscopic" with a higher affinity for water. But hydraulic fluid can collect issues over the years. Dirt gets in and settles out as well as o-rings and the inner lining of rubber lines breaks down. Moving parts and bits of metal.
I had replaced both fuel pumps and the process involves checking high RPM pump pressure. Run up engine to 2700 while measuring fuel pump pressure, which requires full and tight braking which I did not have. Also, comparing, I still have my 182 and definitely my "recently" acquired 337 (hah--wrapping up a 15 month first annual) was noticeably softer.
So off to a brake system re-do.
My first problem was leaking swivel joints. Inspection revealed gummy sticky hydraulic fluid in the landing gear swivel area. I reported in another thread finding a repair facility for this hard to find and fix part: Robair Repair in Silverdale WA (
www.roabairrepair.com). Sent them out, got them repaired and pressure tested, reinstalled.
Because my swivel joints had leaked, system was low/out of hydraulic fluid and had air in it. I filled the system in retrograde applying hydraulic fluid under pressure at the nipple on bottom of caliper at wheel, pushing fluid upwards to master cylinder limiting the chance of leaving air in the system.
Still weak brakes. So next step was to replace o-rings in master cylinder. Fairly easy though does require being on your back under dash and I did have to loosen the left rudder pedal to allow it to flip forward. When master cylinders disassembled, gunky dark thick hydraulic fluid and sediment had accumulated in master cylinder bottom. Getting to your point, this is where you can see the bad stuff from leaving fluid in there long time. (My A&P was concurrently complaining of gunk and nasty fluid in the gear system of another customer in the shop that had been closed for years and requiring disassembly for cleaning.) While at it, I replaced the "rubber" lines connecting master cylinders to parking brake valve as these are oft overlooked and mine seemed pretty hard (brittle) at risk for failure at an inopportune moment.
I have an E model 337 and it's serial number involves the brake lines looping up in the passenger sidewall, a location where where I had the complication of of air bubble trapping despite my best intentions. Bleeding the system required cracking the lines at several locations to be sure the air was out of it. If you have softness bleed again and again to be sure air is out. Seems like it took a hundred pumps and nut "crackings" to get all the air out.
On test day brakes were firm at first but the right went soft. Hydraulic fluid poured on ramp. I had left the "lifetime" silicone stainless steel lines in place from previous owner in the landing gear well. But one in the right landing gear well was oozing at the nut. I suspect a hairline crack there where nut meets the flexible line. Replaced and bled and bled again; problem solved with bilateral firm brakes. Lesson here is there are no "lifetime" lines and all old lines are suspect. If you have a suspicion and have the system open, replace lines anyway so you only have to bleed it once.
While very functional I still have some brake softness compared to my 182 and a next project is the o-rings in the calipers. Replacing those in my 182 helped alot.
Short story: attention to the swivels and lines in landing gear well; alot of motion there. Do o-rings in master cylinders. Much bang for the buck and after replacing those 90 cent o-rings one of the most satisfying feelings to have brakes so improved. Master cylinders also where sediment and gunk collect. A simple back flushing didn't move any of it and the cylinders had to be manually cleaned. (Notice how sticky your hydraulic fluid can get.) Be suspicious of any air in the system with residual softness after the service. Also be suspicious of air getting into system with incompetent brake swivels. Don't trust old hydraulic lines no matter how good they look. Don't forget o-rings in calipers.
From what I recall reading in past, skydrol is high fluid performance overkill for the flying we do. After you have wetted yourself with plenty of 5606 on this job you will be happy with it.
Now, how about a process for cleaning and flushing out the landing gear system...