Brake system flush?

[mshac]

My BMW requires its brake fluid to be changed every two years due to potential moisture in the system. BMW claims failure to do this can lead to mushy brakes and other issues.

Since brake fluid DOES absorb moisture, who here has ever flushed their aircraft's brake fluid purely as a preventive measure? I don't recall seeing it discussed much, if ever.

My P337H has fresh brakes, but the effort required for full braking effect is rather high IMHO. I'm considering flushing the brake system with new fluid to see if it helps. I couldn't find any log entries related to flushing the brake system, so I wonder if its ever been done before.

Should be very simple - Open the bleeder valve on the brake caliper, and pump out enough fluid with the toe brake to empty the reservoir. Refill the reservoir with new fluid. Pump brakes and refill with new fluid as required until the new fluid is coming out the bleeder valve. Color will be different, so it should be easy to tell. Close the bleeder valve, then repeat the process for the other wheel. Put a small tube on the bleeder valve so the old fluid can be directed into a container for proper disposal, rather than all over the hangar floor. Brake fluid WILL eat through painted and epoxied floor, so be careful! Its no good for your skin either.

Any thoughts on using Skydrol rather than traditional 5606 hydraulic fluid?

Comments are welcome, I'll report back here with my findings after get a chance to do it.

[kbecker]

I don’t think 5606 hydraulic fluid and DOT 3 brake fluid have the same hydroscopic characteristics.

[wslade2]

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...

[JAG]

Great posts - I will just a few thoughts.

NO to Skydrol...not compatible with our power packs or seals. Besides, that stuff is even nastier to paint! I have worked with the stuff for many years, give me 5606 any day!

Changing the oil in your brakes - yes, it is a great idea. I do this about every 5 years on my aircraft, and it is really simple. I have one of those pressure tanks from ATS - fill it with fresh fluid. Take some of that plastic line you use for pitot-static lines, and screw in a NPT fitting into top of master cylinder (I think 1/4" NPT) and about a 24" piece of line into a water bottle. Open up your bleeder and pressurize from the brake - you will see the old oil come up through the hose and into the bottle. Once it is running nice and clean keep going until you fill about 3/4 of the bottle, and shut it all down. You have just changed the oil in a brake and brake line, and bled the brakes at the same time. It does not hurt to reseal the brake calipers and clean them once and while, including the master cylinders too. After about 10 years, they get a bit dirty inside and can use a good cleaning and new seals.
Jeff