Rethinking or overthinking the bleeding process

[tony-muscle]

I understand the traditional bleeding process where you have someone push the brake pedal while you open the bleeder, let fluid come out, close the bleeder and release pedal. Repeat until no more bubbles come out. There may be more details and some slight variations of this method but in general that's what it is. Now, let's say you have your system all bled but you change the master cylinder. Assuming you bench bled the master, you will lose just a little bit of fluid at the connection between the time you disconnect the old one and connect the new one. That being the case, you will have a small amount of air sitting up in the tube by the master cylinder. This is where I am trying to figure out why the system needs to be bled in the traditional way after replacing the master as described (assumes system didn't have air before, master was bench bled and master sits at the highest point of the system). If there is just a little bit of air in the tube by the master, wouldn't it make sense to just let the air bubbles vent up through the master instead of pushing them down the whole system until they come out by the brake bleeder? Actually, if you were to bleed the traditional way, you will think you don't have air in the system after a few pedal strokes because it should take a bunch of strokes for the air to travel through the whole tube down to the brake bleeder.

What are your thoughts?

PS: I just replaced my master and I am at this step.

 

[Hemikiller]

I've had very little issues with bleeding after the master cylinder was replaced. I typically cap the lines as soon as I remove them to minimize fluid loss. When threading them back in, most of the air escapes anyway as fluid from the M/C dribbles into the void. You can also have someone hold the pedal while you crack the line at the M/C. Another option would be to pressure bleed from the wheels. I just replaced the entire brake system on my '07 Silverado and used this method. You could probably get away with doing only the passenger side front and rear.

 

[jowens1126]

Changing the master is one place where a reverse bleeding pump is handy. You can get a bleeding kit that has what looks like a hand vacuum pump in it, but it runs as a compression pump that will pump fluid into the brake bleeder and up thru the whole system and into the reservoir. That way anything on the top end that has air in it gets flushed back into the reservoir. I used to have one of these and used it several times on my bronco. It worked pretty well, It even got air out of my ABS hydraulic unit. I don't remember where I got it. I only remember it was on GEARZ.

 

[tony-muscle]

Changing the master is one place where a reverse bleeding pump is handy.  You can get a bleeding kit that has what looks like a hand vacuum pump in it, but it runs as a compression pump that will pump fluid into the brake bleeder and up thru the whole system and into the reservoir.  That way anything on the top end that has air in it gets flushed back into the reservoir.  I used to have one of these and used it several times on my bronco.  It worked pretty well, It even got air out of my ABS hydraulic unit.  I don't remember where I got it.  I only remember it was on GEARZ.

Yeah. That's what I am thinking. Another way I thought is to compress a caliper(s) in the rear and in the front to push some fluid back.

I am not trying to cut corners but just doing it smart. Whatever I do, I will still do a traditional bleeding process afterwards.

 

[Carolina_Mountain_Mustangs]

I believe brake fluid is the only fluid in a vehicle that will absorb water. Motor oil will not, PS not and auto trans. not.

You need to flush the system on occasion just to keep fresh fluid in. Yes you can get the test strips to look at the moisture level but doesn't take that long to bleed them.

I use a cheap Harbor Freight vacuum hand pump but works fine. Just be sure you hook the pump up to the correct side of the catch can so you do not suck fluid into the pump. It will pull over 20 on the brake system. Anything as safety related as brakes I treat with caution.

I had the single piston master cylinder blow on my 65 vert on way to work one morning. Wife was in passenger seat and I had cup of coffee in my right hand with no cover. It is a 4 speed. Came around a curve and school bus is stopped. I hit the brake and boom right to the floor. I threw the coffee cup in the floor on her side grabbed the shift and crammed in second gear. Reached down pulled the ebrake and still going to hit the bus. I swerved to the right ditch was shallow and went about 10' past the rear of the buss and stopped. WHEW. Got it backed up went home and got another vehicle.

 

[tony-muscle]

I believe brake fluid is the only fluid in a vehicle that will absorb water. Motor oil will not, PS not and auto trans. not.

You need to flush the system on occasion just to keep fresh fluid in. Yes you can get the test strips to look at the moisture level but doesn't take that long to bleed them.

I use a cheap Harbor Freight vacuum hand pump but works fine. Just be sure you hook the pump up to the correct side of the catch can so you do not suck fluid into the pump. It will pull over 20 on the brake system. Anything as safety related as brakes I treat with caution.

I had the single piston master cylinder blow on my 65 vert on way to work one morning. Wife was in passenger seat and I had cup of coffee in my right hand with no cover. It is a 4 speed. Came around a curve and school bus is stopped. I hit the brake and boom right to the floor. I threw the coffee cup in the floor on her side grabbed the shift and crammed in second gear. Reached down pulled the ebrake and still going to hit the bus. I swerved to the right ditch was shallow and went about 10' past the rear of the buss and stopped. WHEW. Got it backed up went home and got another vehicle.

You are correct. Brake fluid is hygroscopic, which means that it will attract water molecules. As a background, I have flushed my system twice these last two years. Once after replacing brakes at all four corners and some of the tubing, and then again last year after replacing the middle tube from the engine to the rear. I am not trying to cut corners, I am just trying to use some logic in regards to the air that is near the master. A shorter path would be for them to go up through the master not down through the whole system. I will still bleed the brakes the traditional way.

 

[c9zx]

The Motive bleeder works well for flushing and bleeding brakes with one person. Chuck