RV-4 Brake Servicing

Had an interesting experience this weekend as I changed the brake lines on our RV-4 (which I did not build)...Brett at Bonaco did a great job of building new braded lines, and I dutfily built the requisite pump assembly using appropriate fittings and a garden sprayer, shamelessly copying the design elsewhere in the forums...

...After a couple of hours and recirculating over a gallon of fluid through each side from the bottom up, no pedal pressure (at least I was positive I flushed all of the 5606 from the system and replaced it with 83282!)--lots of head scratching and well meaning advice. Pumping the brakes down like an automobile? No pedal pressure and more head scratching. Call to Van's Tech Support--the master cylinders in the RV-4 are upside down, bleeding can be sporty...total head scratching time, well in excess of four hours...

...Disconnected fancy spray bottle from bleed screw, removed the line and fittings and plumbed it to the overflow hose fitted to reservoir. Pumped up the pressure, opened the valve, put a container under the bleed screw and bled each side in less than a minute.

"Bottom up" bleeding works great when the cylinders are right side up, but if they're upside down, top down may be warranted/required. You can also save the expense of the fitting designed to fit the bleed screw, a clean shallow container is all thats required. When the reservior is being fed by a pressurized system, there's no doubt when air is escaping at the bleed screw, the fluids spits out rather than flowing smoothly. The stock Van's reservior takes a 1/8" NPT fitting, and an appropriate length of clear tubing is all that's required to hook it up to a pump.

It was also interesting that with the pressurized container hooked to the reservior, the pedals where nearly rock hard--like having power brakes. When the hose was removed and the vented cap replaced, the pedal pressure returned to normal.

Cheers,

Vac

Glad it worked for you but

I would go ahead and flip your MCs over when you get a chance. I did mine and wondered why I didn't do it 3 years earlier. The problem with top down bleeding is unlike a car, the bleed screw on an aircraft caliper is on the bottom. Air bubbles rise. So there is a chance to that air will collect at the high point in the brake cylinder. Also, if you brake (pun intended) down somewhere and need to bleed the brakes, you can borrow simple bottom up bleeding equipment from anyone. Once you flip the MCs over, you can bleed your brakes in minutes with a pump style oiler and a piece of clear tubing. I know, I fought the same problem you had for a while. I came up with a different solution the worked but was a PITA. Finally I flipped over the MCs and brake bleeding is now a non-event.

Brake lines

Did you have to measure or does bonaco have a standard kit?

Measured. Just used a string. The lines are -3 sized. You'll need to check the AN elbows on your calipers/master cylinder/connectors so that you can let Brett know what size fittings to install on the ends.

RV-4 master cylinders upside down

Thanks to Vac: I couldn't bleed my right side brake; rebuilt the master cylinder; nada! I was tearing out what little hair I have left until I read Vac's post.

I was able to remove the right side master cylinder, invert it, and bleed the right side from wheel up to the reservoir, while it was inverted. Rock solid pedal afterwards.

Barney, in Memphis
RV-3 & -4 flying

Had same problem, very madening....solved.

After flushing a similar amount of fluid, thinking the seals in the MC were bad, wasting hours rebuilding/flushing....just not making any sense.

I finally unbolted my brakes, turned them upside down so the bleeder valve is facing up, pressure flushed/bled them from reservoir up top, bolted back in place, done in minutes.....

What a PIA ! Must be something about the length of tubing is just right to move the bubble from the piston to the MC, back and forth???

You can pressurize top down (i.e., hook the garden sprayer up to the reservoir) and have an assistant pump the brakes and bleed it just like a car using a small (clean) container to catch the fluid under the brakes. This saves the effort of having to disconnect the cylinders when you want to bleed. It's painfully obvious when air is working its way out of the system. Option B is to simply flip the cylinders (either temporarily or re-mounting them permanently) as some other folks have done allowing the classic bottom up bleed.

Cheers,

Vac

Bleeding the RV4 brake system

Yup, VAC has come up with the correct answer. Using a pressure bleeder we tried to bleed top to bottom then bottom to top. No air coming through each attempt but still, no pedal at all. We left the pipes tight and attached to the master cylinders on the pedals, removed the cylinders and inverted them to a plunger up position, carried out a bottom up bleed and bingo, perfect brakes after removing an awful lot of air!!!
Thanks for the tip
Paul (G-RVNS)

I wonder how many other 4 owners have gone through this trial!

I did much the same with similar results. I finally had to give up and drag an A+P buddy out who repeated the same process without result. (at least it wasnt JUST me)

Pressure bleeding from the top was easy enough. Gonnah h look at flipping the MCs right side up when I get home, would this require any modification?

Cheers