Gravity bleeding brakes

faof2 · Post by **faof2** » 01 Jul 2013, 19:38

1991 Volvo 240 station wagon, auto. trans, A/C, regular(non-ABS) brakes, 110,000 miles.

I reviewed several posts in the archives about gravity bleeding brakes, and no one seemed to find fault with this method. However, I would like to hear more discussion about this topic, so I have posed several questions for comment:
1. Are there any risks with this method?
2. Can air get into the system or get trapped somewhere using this method?
3. Will the "dirty" fluid be expelled as thoroughly as it would pumping the brakes?
4. Is this method suitable for the times you only want to change the fluid?
5. Is it suitable for bleeding after changing components such as calipers, master cylinder, hoses, etc.?
6. Will this method eliminate the need to "bench bleed" the MC? (I suspect not)
7. If one forgets to keep the reservoir full and air gets in, will continuing this method assure that all the air goes out?

Post by **JDS60R** » 01 Jul 2013, 21:06

1. yes incl running dry
2. Not if done properly
3. yes
4. yes
5. No
6. No, but most systems will allow it to work.

difflock54 · Post by **difflock54** » 01 Jul 2013, 21:33

Aside of the Master Cylinder situation, why would this gravity bleed system not work after replacing say callipers, flexible brake hoses etc ??

Post by **JDS60R** » 01 Jul 2013, 22:38

It usually does.(99%) But on certain systems the brake line is routed in a way that does not only have a downward slope towards the car. Sometimes ( for reasons hard to understand by mechanics) they run the line up and then down again leaving a place where air can be trapped. As we can't clamp off the hard line while replacing flex lines some fluid leaks out and a bubble can get in (rarely) or the mc empties allowing air in. ( Always suggested to clamp the old line after loosening at both ends, remove caliper and mount new caliper and line. This way you make a fast switch from one line to another avoiding a dry mc.)

It seems logical to say the downward pressure of the fluid combined with the surface tension against the line will easily overcome any possible place where air could be trapped. And that is right 99% of the time Unfortunately, in reality, sometimes that bubble won't move without a little push or extra vacuum on the line.

There are lots of cheap ways around it including reverse bleeding. You can take a syringe of brake fluid hooked to the open bleeder and literally push the bubble back up into the MC. Or you can install a tire valve into an extra MC cover ( with air vent sealed )and make your own $4 pressure bleeder.

Remember it usually does work without help and can be avoided if the lines are kept full.

Post by **jimmy57** » 01 Jul 2013, 23:22

The simple way to stop brake fluid loss when having lines open for repairs or parts exchanges is to depress the pedal part way. The fill ports into the cylinder are at rear and if the piston is pushed past fill port then no fluid leaks other than what can run out of lines. Usually that is a small amount since no venting occurs and the small diameter of metal lines isn't likely to let fluid and air pass each other to vent from the open line(s).

If I don't have an adapter for pressure bleeding then I do it the old one-man way. Open the bleeder screw 1/4 turn or a bit less and then slide a hose onto the bleeder screw. Have the end of hose at bottom of a container and the container fastened so it keeps the hose in place. Baling wire to something nearby works well. Pump brake pedal 6 times and then close bleeder and check pedal firmness. Check fluid level and top up and repeat if needed. If flushing I suck out the reservoir, add fresh fluid and then pump a dozen times and checks to see if fluid coming out is clean. GO to the next caliper and repeat while keeping the reservoir topped up every 12 strokes or as many strokes as you figure out is safe for that car. Not all cars have the same displacement of fluid per pedal stroke.
I use clear vinyl hose and 16 oz drinking water disposable bottles usually so I can see bled out fluid.

Post by **jimmy57** » 01 Jul 2013, 23:28

By the way. I'm about to flush the system on my car and use some high performance ATE DOT 4 ++ fluid.
It is illegal in US due to color (Ebay purchase). I sure hope I don't get busted. I'd hate to serve time in the Federal Penitentiary where pillow and furniture tag removers are sent. I'm thinking that is a way mean bunch of hombres.

Post by **abscate** » 02 Jul 2013, 10:29

We will come visit you in Supermax, but the guys who removed the autostop on the lawn mowers are the ones you want to avoid...

difflock54 · Post by **difflock54** » 02 Jul 2013, 19:52

Thanks to JDS60R for the very logical explanation to my query above regarding the gravity systems use when fitting new brake hoses or callipers.
It makes sense especially as the front brake hoses particularly, generally have a midpoint fixing bracket against the suspension strut to offer intermediate support and the hoses do commonly loop up and over in many vehicles, not unique just to Volvo's either, allowing for a potential air lock as you described.