Bleeding brakes by gravity...easy?
 

Now that I have recovered from the flu, I'm back on the Vette. Here is the refresher notes on my '67. Did rear suspension work, this required the removal, and subsequent replacement of the rear caliper. I mistakingly let the REAR portion of the dual MS go empty. Everything is done now except the bleeding. I have the rear of the car higher than the front. I have cracked ALL the rear bleeders a full turn + a bit more. I have clear tubing attached to each bleeder valve, then leding to small bottles, and the REAR of the MS is filled. This was done an hour and a half ago. ZERO fluid has reached the calipers. Using gravity, as I understand it, you do not touch/pump the pedal. So, now what? I have time! :crazy2: Dennis

Gravity is much less effective when there's air "early" in the system. There's not enough pressure from gravity (or not enough fluid above, depending on how you want to look at it) to overcome the air pressure inside the lines. Closer to the calipers, the increased pressure (because more fluid above is pressing down) is enough to get it moving.

Has the fluid level changed at all? 90 minutes is definitely enough to see if it's doing anything...I'm guessing it hasn't moved at all. Edit: As mentioned below, you have to get it started. Mityvac's kit is pretty good...I don't know if the cheaper copies are as durable.

you have to get the fluid started down the lines.

I would suggest a friend or a Vacuum source. If you don't have a small vacuum pump, you can use the manifold port on the engine while it idles as a vacuum source, just be sure to use a catch jar. - it's a 327 vacuum pimp.

I just did it at a friends house. 15' of plastic hose from home Depot...and a 24oz jelly jar.

That's the theory anyway, and in many cases it works. In your case, air in the reservoir port to the rear side of the MC may be bridging the port so fluid can't flow into the MC. It's a small hole. Sometimes it takes a pump or two on the pedal to get the fluid started. Sometimes all the air can be purged by gravity bleeding. Sometimes not. Surface tension in liquid is a funny thing; sometimes it just turns out to be necessary to put a little force behind the fluid to clear all the air bubbles.

I have always gravity bled with success but had the car level, how is the fluid going to go to the back if the back is higher? In addition you dont open all the bleed screws at the same time.

I don't believe gravity bleed is going to work here unless the MC is bench bled first to get the fluid going as stated above.

It's intended to help the air to exit at the highest point, the bleed screws. I have viewed two or three videos showing successful bleeding on multiple calipers. :thumbs: Dennis

It has been my understanding from past info here, that bench bleeding is not necessary since this is a dual MS. It just needs the fluid to start moving by gravity.

As suggested by BLUE1972 and Godhollo, I will put my MityVac to work.I'll start on the furtherest bleeder. Nothing to lose. Thanks guys, I keep you posted. :thumbs: Dennis

:iagree: Gravity is not going to work here unless the master cylinder is higher than the calipers. If you choose this method, you'll need to be sure the master doesn't go empty in the process or you will reintroduce air into the lines.

Having said that, some gentle pumping of the pedal in its' current position (calipers higher than master) will get fluid flowing and should begin to fill the lines and ultimately start leaking from the calipers bleeders. As each bleeder starts leaking fluid, shut the bleeder until fluid has flowed from all of them. Most of the air is now out of the lines and calipers. At that point I would commence a manual bleeding by having an assistant pump the brakes while you bleed each caliper individually. Once all the air is out, you should be good to go.

Good luck... GUSTO

MikeM is correct, unless you bench bleed the mc you get an air lock that prevents the fluid from leaving the mc.

However, you can bench bleed the master on the car. If you are only doing the rear circuit, take an old piece of brake line and leave the flare and fitting on one end that will screw into the mc port. Cut the other end off leaving enough line that you can bend it into the reservoir about 2/3 deep into the fluid. Then pump the brakes until no more air comes out. Put a rag under the fitting as you disconnect it and as long as the reservoir doesn’t go dry by the time you reconnect your brake line the mc is bled. On my brand new system (mc, lines and calipers), it took about 10 minutes for the fluid to appear at the right rear caliper bleeder. It should not take much longer than this if your mc is properly bled.

It’s a ‘67. What about using the 2 bleeder valves on the MC?

Steve

It's a replacement MC without bleeder valves. Original is tucked away. I actually thought the other day about sending it out for rebuild by White Post. :cheers: Dennis

I always do gravity bleed. When I get one that doesn't want to cooperate I place my palm over the mc to seal it and push on the back of that hand with my other hand and get fluid moving, If that doesn't work I'll push pedal a couple times. The only time I bench bleed is if mc is new.
Chip

That's a neat trick!:thumbs:

PS. The only MC's I've ever had to bench bleed are the dual chamber style. Not the single.

I don't think you can rely on gravity bleeding alone to do a complete job if there is already air in the lines. It's great for replacing fluid in a bleed system or maybe for bleeding a single caliper that has been opened up and the line immediately plugged to keep fluid in the system. Even with vacuum bleeding and manual bleeding (press pedal - hold - crack bleeder - close bleeder - release pedal, it's common to have to tap the calipers with a hammer or mallet to get the last of the air out of a caliper.

Air doesn't care if it's expelled from high, middle or low point in a system. What is important is to have the source of the fluid higher than the point where the air is exiting. Lower the rear of the car and get the fluid moving with a few pumps on the brake pedal. Gravity will then do the rest, one wheel at a time.

True, but what makes a real difference is having the bleeder screws as vertical as possible and at the highest point of the caliper.

https://cimg4.ibsrv.net/gimg/www.cor...e60e9d5d6.jpeg
Rear Wilwood caliper on my coupe gravity bleeding. The inside bleeder is complete, the outside still has air.
https://cimg5.ibsrv.net/gimg/www.cor...e94ccece9.jpeg
But to completely bleed them, I removed them and hung them vertically. I bench bled the mc on the car and then gravity bled the entire system which is completely new from the mc to the brake lines to the calipers.

I have a half inch thick flat steel plate tapped for an air fitting that I clamp to the top of the MC with a c-clamp and a thick rubber gasket. i set the pressure regulator on the compressor to the lowest setting- 10 psi or so and pressure bleed. I just have to be careful to watch the fluid level in the MC.

John - So, basically, you have a homemade Motive pressure bleeder but without the fluid reservoir.

Steve

This^^^. Fluid won't flow uphill. Law of gravity in play here. Get the car back level, fill the reservoir, have a cup of coffee, and sit back. Do one caliper at a time, not both at once. Been doing it this way for decades and it works just fine....if you do it right.

Bench-bleed and then pressure-bleed; whole process takes less time than it takes to remove the rear wheels. :thumbs:

https://cimg5.ibsrv.net/gimg/www.cor...42661c0d18.jpg
https://cimg7.ibsrv.net/gimg/www.cor...2083a44e61.jpg