What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

RV-8 low pressure plastic brake lines

rv8ch

Well Known Member
Patron
I have not been able to fix this leak for some reason. Left leaks, right does not. Tried both the plastic insert and the brass. Cut the hose to ensure a straight connection. Made sure it's clean. Very strange.

RV-8-F-69-F-04x02-Brass-Elbow-Assembly-DWG-83.png


I've read lots of threads where people write "no problem with mine leaking" or "I replaced them with real hose."

I think I will replace them. Seems like it would be better to "Y" them just under the panel, and only bring one hose to the reservoir - anyone see an issue with that?

DWG-83-RV-8-Plastic-Low-Pressure-Brake-Lines.png
 
You sure it's not the NPT threads on the fitting? These need to be tighter than most are willing to do and require a good sealant. I have one that leaks a drop every month or so. Keep putting it off, as I don't want to bleed again.

Larry
 
Better stuff...

I wasn't a fan of the brass ferrule stuff that Van's puts in the kits. For my second RV-7 I used Parker Hannifin Prestolok PLP Push-to-Connect fittings for the low-pressure brake connections.

Parker Hannifin W368PLP-4-2 "Male Y Connector", 1/8" NPT to 1/4" dual tube, at the Firewall/Reservoir (Qty 1).

Parker Hannifin W369PLP-4-2 "Male Elbow Swivel", 1/8" NPT to 1/4" tube at the master cylinder "input" side (low pressure) (Qty 2).

Good luck!
 
not the npt threads

You sure it's not the NPT threads on the fitting? ...
I thought the same thing at first, but unfortunately it's leaking just between the plastic tube and the brass nut. The fluid collects on the top of the nut. Hard to see the fluid with this picture, but you can see where I'm talking about I believe.

UNADJUSTEDNONRAW_thumb_5cf5.jpg


I think before replacing the hoses I'll take the whole assembly out and look at it closely to see if I'm not missing something silly. I just drained the fluid so should be pretty quick.
 
I thought the same thing at first, but unfortunately it's leaking just between the plastic tube and the brass nut. The fluid collects on the top of the nut. Hard to see the fluid with this picture, but you can see where I'm talking about I believe.

UNADJUSTEDNONRAW_thumb_5cf5.jpg


I think before replacing the hoses I'll take the whole assembly out and look at it closely to see if I'm not missing something silly. I just drained the fluid so should be pretty quick.

Ok. Make sure you are getting those tight enough as well. When done properly, the ferrule will compress around the tubing and this operation is critical for preventing leaks. These also need to be tighter than those are used to tightening 3/16" bolts. It takes a decent amount or torque to compress that ferrule.

When you take them off, check the ferrule. If you can't see that it created a slight indentation into the tubing, you are not getting them tight enough. Pretty much every refrigerator in america has this style fittings for the ice maker. They are very reliable when assembled correctly.
 
Last edited:
FWIW, I used the brass Parker 63PT inserts rather than the plastic. Never had a leak. On the pipe threads I used Pneumatic/Hydraulic sealant (like Permatex 54540). I figured if it was good enough for Caterpillar tractors, it was good enough for my airplane.
 
Last edited:
I also have had trouble getting these low pressure lines to keep from leaking - slowly. I have come to the conclusion that there are two types of low pressure plastic lines terminating in brass compression fittings: those that leak, like mine always did, and those that don’t leak now - but will pretty soon.... This is (of course) untrue, but I believe these type fittings take a certain amount of expertise to install correctly, and I was never able to get it right. It’s very important to have a smooth straight cut on the tubing, use the correct tubing insert, and make sure you hold the assembly fully inserted tightly in the fitting, and straight before tightening the nut. Otherwise the brass double tapered ferrell will get slightly crooked/tilted and will either leak now, or leak later, but it will always leak if not installed correctly. I needed a more fool proof design. I found the nylo-seal all plastic fittings and lines to be fool proof and also the push on fittings from safe-air with their nylon tubing- low pressure only. Never had a leak with either of these. For the high pressure side of brake lines, I only use AN fittings with #3 or #4 braided hose and aluminum hard line - never nylon.
 
Last edited:
Back in the industrial days, only brass or stainless inserts used on airbrake tubing---like those used by Vans. THe compression action of the nut against the ferulle will try and distort the hose it there isnt something there to support the ID of the hose.

OH----and when we use teflon for the reservoir application like Mickey posted, we use a 45* hose end on the 'run' side of the tee--the port that comes straight out, we use a 90* hose end on the branch side. Both hoses converge together near the longeron, and route to the master cylinders.

Tom
 
Ferrule position

I think it is important that the brass ferrule is not too far onto the plastic tube. Both sides of the ferrule must contact the fitting. If the brass ferrule is too far on the plastic tube, then only one side seals against the fitting. So I would make sure the ferrules are sealing on both sides.
 
I made the Braided SS switch..

I was a huge fan of the plastic lines when I built my -4 because they were simple, light and easy to DIY the length and routing. However, after 10 years of service, I found myself continually tightening, snipping and re-furreling the ends to keep from having leaks ,mostly at the high pressure caliper end. The final straw to the demise of my plastic lines was loosing my left brake on the departure taxi from OSH this year (made it back to NC with just the right brake). I have switched over to all SS braided AN fitting lines, and hope to never have any more issues. I never did have any leaking of the low pressure side, but mine were all plastic fittings as well as lines (no brass other than the ferrule. I believe time, temperature and other environmental subjection take a toll on plastic system and eventually they become BER.
 
After 2 years of futzing around with the plastic lines in my 9A trying to resolve the same persistent leak, I gave up and had Tom make me a set of real hoses for replacement. No problems since. The plastic line and ferrule setup is not a good material choice for that application, IMHO.
 
Plastic Lines

I am one that has had leak-free plastic tubes in 860 hours of airframe time.

I think much of the success of getting these to be leak free depends on ensuring that the ends of the tubes are cut very straight and also ensuring that the tubing, ferrule, and fittings are clean and scratch/nick free prior to assembly. In my opinion, throwing braided noses I place of the small diameter very low pressure side of the system just adds unnecessary weight and cost to our aircraft.

Skylor
 
Tubing Size

Make sure the tubing is 1/4" O.D. and not 6mm. Sometime a supplier will substitute 6mm (?.236") for 1/4"! I've had this happen to me with industrial equipment. Just a thought.
 
Fixed

Thanks for all the hints about how to solve this - really good info in this thread.

The way I solved it was to use the Steinair plastic tubing cutting tool to make a perfectly straight and perpendicular cut on the tubing, clean everything very well, and put it back together carefully. Seems to be holding now, I'll of course keep an eye on it over the coming days to make sure it's holding.

https://www.steinair.com/product/plastic-tubing-cutting-tool/

P-510-CUTTER-TUBING-DSC09319-L.jpg
 
Back
Top