• Hi Guest!
    You can help ensure that British Car Forum (BCF) continues to provide a great place to engage in the British car hobby! If you find BCF a beneficial community, please consider supporting our efforts with a subscription.
    There are some perks with a member upgrade!
    **Upgrade Now**
    (PS: Subscribers don't see this gawd-aweful banner
Tips
Tips

bleeding master cylinder.

Share this page

bobmga62

bobmga62

Jedi Hopeful
Offline
Okay..
continued reference to my soft clutch

I have replaced the Clutch slave, the flex pipe and the complete master cylinder, when I took the master cylinder out I could see that the push rod was damp on the exterior (of the clutch) so I assume that is the culprit.. weeping of the fluid within the cylinder

but..

now I cannot get the system to bleed properly... though I had thought I had bench bleed the system correctly, by putting the cylinder on a vise and pumping fluid through it..

the front brakes are grabbing, the rear brakes don't even accentuate,, and when I open the bleeder, they just drip a bit, even with pressure.

and when I do the clutch, and I open under pressure, I get no fluid..

sounds like I got one heck of an air pocket in the master..

Is there a way I can eliminate the bubble without taking it all apart again..

thanks

r
 
Ezi-Bleed would be my suggestion.

As for the rear brakes, you did change the flex hose to the diff, yes?
 
I use a broom stick to wedge the brake pedal to the seat all the way down. Then, I will shut the bleeder valve, and very, very slowly release the brake pedal. This way, the brake fluid from the resevoir will fill the MC. When the brake pedal is all the way up, then open the bleeder valve again and press the brake pedal all the way once more and use the broom stick again, and repeat a few rounds. Usually, after about 3 rounds of this exercise, the pedal will get stiffer...

For the clutch slave, I use a 4" G-clamp to clamp the push-rod all the way in. then, open the bleed valve, and pump the clutch pedal all the way down and hold it with a broom stick. then shut the bleed valve, and again release the clutch pedal very very slowly....try pumping, holding, close bleed valve and releasing slowly a few times...the clutch pedal will get stiffer as all the air in the line and slave is driven out. Don't forget to remove the G-clamp once the pedal is stiff....
 
Grab a relative and enlist them to do EXACTLY as you instruct... Have them push the pedal as you open the various bleeders.

YB!!! You apparently live alone. :jester:
 
Actually, i live with my 2 kids but they are not around to help when I am working on the car. Besides, they hate getting grease under their finger nails...so they tend to stay away from dad's toy. Daughter drives an EVO, and when she saw what was under the MGB hood, she was doubtful if it will run at all...
grin.gif
 
Re: bleeding master cylinder.continued

I have replaced lines, slaves, flex lines eetc, still no action with the MC.. both pedals are soft. have used a mity vac and still no luck...this is going on for almost a month now.. urgghhh.

this sounds interesting, but, I do not quite understand your open/close ratio...

By my understanding, you push and hold the brake pedal (with a broom stick) with the bleeder valve open and then close the bleeder valve and slowly release the the brake pedal..this sounds to me that any fluid in the caliper may be drawn back into the MC???


I use a broom stick to wedge the brake pedal to the seat all the way down. Then, I will shut the bleeder valve, and very, very slowly release the brake pedal. This way, the brake fluid from the resevoir will fill the MC. When the brake pedal is all the way up, then open the bleeder valve again and press the brake pedal all the way once more and use the broom stick again, and repeat a few rounds. Usually, after about 3 rounds of this exercise, the pedal will get stiffer...
 
Re: bleeding master cylinder.continued

Which way did you use the mighty vac - pull or pump. Most times I pull fluid thru the lines, but every now and then when I have trouble, I will reverse the MV and pump fluid back thru the lines. Like DOC asked, did you check/replace the rubber hose at the diff, if its swollen together, you'll have problems.

Marv
 
Re: bleeding master cylinder.continued

Yes I replaced the flexible line for both the clutch slave and the rear brakes at the axle..(the rear was quite clogged..,,its been in place for 45 years...)

I have been pulling BF through the lines...How do you reverse it to push, I would assume you are still working off the bleeder..

thanks

r
 
Re: bleeding master cylinder.continued

Since the bleeder valve is closed, no fluid or air can flow back(more like pulled back) into the MC by the spring retracting the piston in the MC.

The idea of releasing the brake pedal very slowly with the bleeder valve closed is to not let any air pockets cavitate in the line in which case the air will expand and push fluid in the lines back into the MC. You can sit there for years and pump the brake but the air pockket will just be compressed and expand forever. So, you need displacement. You want to push any air or old fluid in the lines/caliper outwards to the bleeder valve not let them flowback into the MC. You want the new fluid in the resevoir to fill the MC, so that it can be pushed out to the bleeder valve.
 
Re: bleeding master cylinder.continued

In shops, we had the occasional vehicle that WOULD NOT bleed fully.
No matter what you did.

We used a disruptive bleed that usually worked.
Took two mechanics.
One in the car, pulled his knee up to his chest, the other opened the bleeder...then the first guy nailed the pedal as hard as he could, and held it to the floor.
Second guy closed the bleeder.
Did that 2 or 3 times, any "hidden" or "trapped" air was OUT.
 
Re: bleeding master cylinder.continued

You reverse the flow of fluid. Take some out of the master reservoir. Then with clean fluid in the mityvac plastic bottle(just as if you had drained it from the slave) and the mityvac hose to the slave connected. reverse the hose on the fitting on the pump. Instead of coming straight out(on some models, forget which plastic, metal, etc. are routed) so that when you pump the pump the fluid is being pressurized by the pump, it will travel up through the slave, the hoses and the master in to the reservoir.. When you see the fluid rise in the reservoir, stop it before it overflows(eats paint) and remove some, keep pumping fluid through. Or stop and check your pedal. Usually gets rid of all air pockets with just one platic mity vac bottle full of fluid.
 
I'm new to this, I've just been given a '77 MGB convertable by a sis-in-law for work I've done to their home. It's been sitting next to the garage not running for a few years but I managed to get it running after lots of web searches for stats. Thanks, some of the info came from this board! I'd like to offer a possible solution to your brake problem. I worked at a very good auto garage in Daytona Beach several years ago and ran into a similar problem. Turned out to be the valve inside was stuck to one side. The valve's purpose is to prevent all the fluid from escaping if a line ruptures, thereby giving you either front or rear braking if the other fails/ruptures...it blocked flow to the ruptured side. Sometimes when this happens it can be difficult to 'unblock' it...should normally be centered so as to allow fluid/pressure to both front and rear lines, don't know if this info is helpful but just trying to help.

Have managed to get my new 'toy/project' running but still need to borrow a strobe and set the timing better. A leaking master brake cylinder is on my 'to do' list also...as well as the clutch cylinder, will probably be posting questions here as I work my way through making it road worthy!
 
I've found that there is sometimes air trapped at the upper end of the system.

Have a helper pump the brake pedal a few times and hold. Then loosen the line that goes into the clutch master cylinder (at the upper end)...and then retighten. You may have to do this a few times.
I often find a ton of air spurts out at the master cylinder end. This will make a mess and may damage paint, so use plenty of rags to protect the car.
 
One of the hiccups with the dual brake systems ('68 or newer?) is that I haven't figured out how to bleed them the traditional way of having someone pump then hold the pedal while you open the bleeder. What happens is that the system has two branches. When the pedal is being pushed both branches are pressurized. Opening the bleeder on one branch relieves the pressure in that branch, but the other branch still keep the pedal from going to the floor and you only get a dribble out of the one you opened. (If your system has a red warning lite it should turn on, too.)

You will either have to pump fluid thru the bleeder back up to the the MC reservoir (watch for overflow!) or suck it down and out of the bleeder from the reservoir. I prefer the "pump" method to avoid cavitation that someone mentioned.

Somebody help me here. If you have a dual brake system and you are only getting fluid thru the fronts or backs, can the junction block thing with the warning switch be shifted so that fluid only gets to the front or back and the other is blocked? If that's what's happening the piston may be shifted & seized.

I hope this helps.
 
Back
Top