bench bleeding M/C-absolutely necessary?

[berge]

do you absolutely have to bench bleed a new brake master cylinder? I have a vacuum brake bleeder that makes the job a breeze, can I just slap the new m/c on without bleeding it and let the vacuum bleeder do it's job?

the car is a 92 legend with ABS, BTW.

 

[Mike_dup1]

What do the instructions that come with the parts say? Follow those. I am no brake expert but have listened to advice from some. They have always told me bench bleed the MC or you will just make more work for yourself. If you don't you will be chasing air pockets for hours if not days and weeks.

 

[Kestas]

I've heard stories where people changed the master cylinder, bled the heck out of the four corners, and still had a soft pedal. I believe bench bleeding is a must to get air out of certain areas of the master cylinder before doing the regular bleeding to finish the job.

 

[berge]

ok.

thank you.

 

[JohnBrowning]

Yes, You should always bench fleed. If it is already mounted on the car you can still do a "bench bleed". You would end up setting it up just like you would if you had it in a vice. You would use the pedal in plave of a long screw driver and follow the rest of the direction exactly.

If you are trying to be lazy do not take short cuts. If you do not have a vice then the alternative of mounting it to the firewall and "bench bleeding on car with the supplied hardware will also work. It is harder to control the travel and speed though so be mindful.

 

[berge]

ok, I got problems.

I followed the instructions with the rebuilt master cylinder, bench bled it, bled the whole car, twice, and I have a pedal that goes very low to the floor. I have brakes, but, the pedal is too low.

what is going on?

 

[labman]

If the pedal stops over a fairly short distance, but just too low, you may want look at excess mechanical movement. The pushrod may need adjusted. The rear brakes could need adjustment. I have never had good luck keeping self adjusters working.

If on the other hand, the resistance just slowly builds as the pedal continues to go down, that is the classic air in the line. If you are not using one of the vacuum bleeders, get one and try it. They expand the air in the line, drawing it out of a high spot it may have collected in. I think they are the best deal for the average person wanting a one man bleeder. They will even bleed the notorious older Ford tilt cabs.

 

[berge]

quote:

---

Originally posted by labman:
If on the other hand, the resistance just slowly builds as the pedal continues to go down, that is the classic air in the line. If you are not using one of the vacuum bleeders, get one and try it.

---

I guess I have air in the system. I have a vacuum bleeder and have used it with great result. This is my first time changing a M/C though. I may have screwed up the bench bleeding part.
can I disconnect the brake lines to the m/c and bench bleed it while it's still on the car. run lines from m/c ports back into reservoir, submerged in the fluid, and slowly pump the brake pedal.
will that work?
I used almost 1.5 litres of brake fluid during the bleeding, even if I screwed up the bench bleeding of the m/c, how could I still not get the air out of it with all that fluid and a vacuum bleeder?

 

[Kestas]

Berge, I'm not a big fan of vacuum bleeding. I made this nice homemade 'hookah' and use mouth vacuum to make it work. 90% of the time I pull air past the bleeder screw threads and get lousy bleeding action. Though vacuum bleeding in theory is beneficial (trapped air gets bigger and is more readily pulled through the system), I just can't get it to work on my cars. How did you get vacuum bleeding to work on your car?

Labman, I know you pack the threads with silicone, so this may be the answer to my problem.

Berge, you may want to consider bleeding using the two-man method, or in a pinch use the one-man method recently discussed in the brake fluid forum on the web site.

 

[TheLoneRanger]

quote:

---

can I disconnect the brake lines to the m/c and bench bleed it while it's still on the car. run lines from m/c ports back into reservoir, submerged in the fluid, and slowly pump the brake pedal.
will that work?

---

Berge, yes you can do that. With a pedal that low you probably didn't get the air out of both sections of the cylinder. Make sure that the (plastic) bleeder nuts are airtight, and slowly press the brake pedal all the way to the floor. This will expell air from the front of the cylinder. It shouldn't take more than a few pumps; just pump it until you don't see any more air bubbles in the bleeder hoses.

 

[labman]

On ''bench'' bleeding on the car, it may be more important to release the pedal slowly, maybe even holding it down a little first, giving the air bubbles a chance to rise up where they will not be sucked back in. Also same thing at the corners.

At the corners, better yet, go with the tedious 2 man (1 wife?) procedure of closing the bleeder on the return stroke to insure no air is drawn back in. That is what I always fall back on after the tubing falls off the bleeder or slips out of the fluid container. Since I don't play with the brakes much, my wife tolerates it. I like the brake pedal goes down once, open the bleeder, close it when the flow stops, release the pedal, and a single pump again.

 

[berge]

thanks everybody.

I bench bled the m/c on the car and then did a gravity bleed at the wheels.
It is now massively better, my wife thinks it's fine, but I know that the pedal is not as firm as before.
there was alot of air at the RR AND LF.
I didn't use the vacuum bleeder because even though I know it works very well, it always pulls air in through the attachment point and all I see is a stream of liquid and bubbles.
this is my procedure, if anyone sees any mistakes I'm making(this is my first M/C change) please tell me.

run lines from disconnected M/C outlet ports(2) back into reservoir.
slowly pump brake pedal all the way to the floor and release slowly until there are no more air bubbles.
reconnect brake lines to M/C.
bleed brakes at wheels.(shop manual shows RR, LF, LR, RF.

thanks everybody.

 

[berge]

one more question, if there is air in the lines or m/c, will it always stay there or will it eventually work its way up into the reservoir and out of the system?

 

[dkcase]

quote:

---

Originally posted by berge:
one more question, if there is air in the lines or m/c, will it always stay there or will it eventually work its way up into the reservoir and out of the system?

---

It will stay. If there's not too much for the brakes to function, you would find it necessary to pump the pedal to avoid it sinking to the floor. If there's too much air, you couldn't ever get a firm pedal.

 

[labman]

Back when I lower on the food chain and the learning curve, I ended up putting a car back on the road with a soft pedal a couple of times. There is the matter of getting to work while you figure out what to do next. In both cases, the pedal firmed up in a few days of cautious driving.

Still, you would be safer with the air gone. Does your ABS have bleed screws? I think my 92 Grand Am did. I never did anything with them since the ABS worked fine, and the manuals never suggested doing anything. I never changed the master cylinder, and was very careful to never let it run dry when I worked ot the rest. I think the shop manual was very explicit about not letting air into the ABS. Even if you properly bled the MC, some air in the line may have slipped into the ABS. Check Chilton's or something better if you can about the ABS.

[ May 13, 2004, 05:09 PM: Message edited by: labman ]

 

[berge]

everything good.

bench bled m/c on the car. the plastic adapters supplied with the rebuilt m/c for the bench bleeding were leaking, I could hear the hissing from one of the ports. poor design. teflon tape to the rescue.
after m/c, bled the car with my trusty vacuum bleeder, used about 1.5L total brake fluid.

pedal nice and firm.

thanks everyone for the help.

 

[Mike_dup1]

Another tip it says in my GM service manual is to start the bleeding from the wheel farthest from the MC and work closer.

 

[labman]

Great! Was the plastic adaptor badly designed or simply have too much flashing or poorly aligned threads? The best design in the world won't survive sloppy execution.

 

[berge]

the ports on the m/c are different sizes. to cut costs they supplied one plastic adapter which kind of fits both ports instead of two adapters which fit perfectly. the bigger port accepted the adapter without any threads showing but the smaller port would only accept about 2-3 threads of the adapter. not enough. air leakage during m/c bleeding.

silly way to cut costs. how much does a plastic plug cost.

 

[AcuraTech]

There is a bleeder screw on the ABS modulator and when you are trying to get air out of the system it is a good idea to bleed that also. Start the car, put a plastic hose on the bleeder and crack it open slowly. Fluid should start rushing out and you should hear the ABS pump run to restore pressure. While doing this just make sure that the ABS resevoir remains topped off so you dont suck air in. Air in the modulator will definetely cause a soft pedal.