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MG Midget and Sprite Technical - trouble bleeding brakes - starting from empty line

Today I finally got around to bleeding my newly installed clutch and brake hydraulic systems.
For both the clutch and brakes I am using speed bleeders (one way valves that let stuff out but not back in).

First I bled the clutch. I used my ez-bleed and pulled the fluid out the bleeders using a vacuum line connected to the bleeder. This worked fantastically (my previous attempt had resulted in airlock).

Then I went on to bleed my completely dry brake system (entire system dry). I should add that I replaced the o-rings in the shuttle valve in the brake light valve assembly (whatever you call that thing). When I re-installed it I am pretty sure that I put it in the middle before installing the switch.

Here is where I ran in to trouble. I had so much luck with the clutch line that I tried using my ez-bleed gadget on the brakes. I started by opening the bleeder on the right rear brake and connecting my ez-bleed to it. I pumped up a vaccuum and then pumped the brakes a little. At first it seemed that some fluid was going through (the level of fluid in the rear reservoir went down a little). and then it stopped. So then I tried the right front. It worked fine (again I used the vacuum bleeder to assist). I was able to easily bleed the right front).

I have been able to get a little fluid out the right rear bleeder but only a few times.

Here is what I think my problem is (but this is a question):

I think that when I started bleeding a dry system I went about it incorrectly. I am starting to realize that if there is an imbalance in pressure between front and rear the shuttle valve will move and shut off the flow of fluid (is this true)? Is it possible that I can't bleed it because the shuttle valve won't permit any fluid to reach the back. I am afraid that my vacuum ez-bleeder trick caused this problem and that I should have just opened the right front and right rear bleeders and gently pumped the brakes a little (in hopes that the shuttle valve does not move).

Do people think that this is what happened? If so, how do I fix it?

Thanks,
Rebecca
R Harvey

unscrew the switch from that shuttle valve unit and shift the valve back to center with a thin screwdriver (the o-rings will prevent any fluid from coming out).

The trick to bleeding brakes with that valve is to use very low pressure. Work slowly and the valve will tend to stay centered.


Norm
Norm Kerr

I did think of that but when I looked in the hold I figured that there was not enough room to do it with damaging anything.

Is it very easy to move the valve? I will try this.

Should I open both the right rear and right front bleeders at the same time and then very gently pump the break (not all the way down)? I am also thinking that I should not use the ez-bleed (the vacuum presure moved the shuttle valve).

Thanks,
Rebecca
R Harvey

Tweaking the valve with a small screw driver worked.
I seem to have completely bled the lines.

However, my imbalance switch seems to be bad (I tested it). I ordered a new one and it does not fit (and it does not look like mine - it is longer). According to the Moss catalog there are two different ones. One is for the original brass valve and the other is for a newer cast iron one. I ordered the one for brass (because mine is brass) but it is wrong.

In this picture, the one on the right is mine. The one on the left is the one that I ordered (and you can see the bag that it came in with the part number):

http://www.pangalacticconsortium.com/cars/MG_midget//imgpages/image101.html

looking at the catalog it seems like I ordered the correct one but it is not correct.

Anyway, I tried centering it the best that I could with a screw driver and went for a test drive. My brake parts are of course not broken in yet (all of them are new).
However, I was expecting the braking performance to be better after a few minutes of on and off hard braking. Is it possible that I have an imbalance that is leaving me with less than 100% braking capability? After I get the correct switch then I expect that I can get it correctly adjusted.

Rebecca

R Harvey

Hi
the switch, and that brass housing don't have anything to do with the balance of the braking system. That stuff is only to turn on the light on the dash if one half experiences a big change in balance (a leak).

Instead, the balance is achieved by the difference in diameters of the rear cylinders vs. the front caliper pistons, and is not adjustable. That is, if one installs the wrong rear cylinders it will be way off, but there isn't any fine tuning possible without installing something aftermarket into the lines.

Can you describe a bit more what you are feeling when braking?
- Is it spongy (air still in the lines)?
- Does it feel like the pedal has to be pushed down with "too much force"? Unless you add a booster to reduce that pedal effort, the brake pedal effort on these old cars can feel like a lot, compared with the (boosted) brakes on modern cars.
- Does the pedal have to be pumped to bring it up to pressure? That usually means that the rear brakes need more adjusting.
-Do the rears lock up way too easily? That could mean that you were supplied with the wrong (early) cylinders from the 4-drum brake system.

Oh, and what year is your car?

hope this helps,
Norm
Norm Kerr

Hm. Interesting. Then how do you explain the fact that fluid from one side or the other did not appear to be able to pass unless I tweaked the shuttle valve to the center with a tiny screw driver (then it was easy to bleed). I am confused.

Anyway, I am definately not locking up the rear brakes.
The pedal does not feel spongy (although I never drove the car before I did this so I can't compare).
I don't need to pump the pedal.

The rear brakes probably need to be adjusted (I just remembered that I forgot to do that). Although they are the minor % of the braking. I am hoping that a combination of rear brakes out of adjustment and new (need a little more breakin) is the entire problem. I just have to figure out what switch I am supposed to have.

Thanks for the reminder about adjusting the rear brakes.

Rebecca
R Harvey

I had this problem with mine. I had to adjust rear brakes then bled as normal but only pumped the pedal with light strokes and do not push pedal to floor or the shuttle will shut off fulid.I used a bleed tube with a non return valve so did not shut of bleed screw to all air gone.
pd cumbers

Howdy,

in answer to your question about the shuttle valve: it doesn't really meter fluid from front to back, actively, like a proportioning valve would do, it just shuts off one side when the pressure drops on that side, to (hopefully) make sure you still have some ability to stop using only the other (non-leaking) side. That is also the reason why it does not turn itself off after being activated, because the system would require repair at that point (to fix the leak before driving any further), if it was ever activated. This is why it is a pain when bleeding the brakes, it can't tell the difference. But bleeding them slowly avoids "awakening" it and it getting in the way.

Let us know how it feels after adjusting the rears.
":oD

Norm

Norm Kerr

Ok. This is what I thought. I am using speed bleeders (have built in one way valve). These things are awesome. After I learned the trick of tweaking the valve to the center with a tiny screw driver I was able to bleed the breaks just fine (pressing the pedal gently).

I will report back after my rears are correctly adjusted (and the fronts are better seated). Next weekend...

Cheers,
Rebecca
R Harvey

Rebecca, I've had problems using a vacuum bleeder or speed bleeders due to air leaks around the threads when they are loosened. I've used a bit of teflon thread seal around the threads (!!! only the threads, none on the tapered seat area !!!) and had much better results.
Bill Young

Bill,

You pose a great question... It wasnt till recently I learned that im one of the few as far as Im aware that use teflon tape/paste on the threads of hydrolics. I thought everyone did it...certianly not so.

why isnt this the norm, I cant imagine bleeding hydrolics without teflon on the threads of bleeders, Id think it would be a nightmare as you would never know if air was being sucked back past the threads or not and id think you wouldnt be able to open the bleeder up far enough to do the job correctly. then add the safty issue of just driving down the road, appeartly it works not to use it as scary as that sounds, but for the 2 pennies it would cost to use the teflon it just seems it would be insane not to use the teflon

just curious why thats not a normal standard thing to do... as it seems just you and I are the only ones that do it.

Prop




Prop

Just FYI, in the factory manual for the 1500, the bleeding procedure involves connecting bleed tubes to the front and rear brakes on one side as mentioned above, and bleeding both together so as not to introduce an imbalance... then doing the same on the other side.

-:G:-
Gryf Ketcherside

Prop, the problem with using teflon tape on hydraulic fittings is that if a small bit breaks loose it can cause failures of seals or block ports in cylinders so be very careful and make sure that there is none around the tapered seats which must be a metal to metal fit. In the case of places were you have steel fittings installed on aluminum parts such as a Mocal oil cooler I like to use tape to help prevent galling of the cooler fittings. I use only tape, no liquid or paste type teflon sealers, just too hard to control where those go or spread.
Most speed bleeders have a coating of teflon on the threads to seal them from the factory, but this wears very quickly and by the second or third use it often allows air leaks I've found.
Bill Young

By the way, this is what the shuttle valve thing looks like:

http://www.pangalacticconsortium.com/cars/MG_Midget//imgpages/image100.html

Rebecca
R Harvey

This thread was discussed between 31/05/2010 and 10/06/2010

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