Brake bleeding issues....Help please...

[brujahstorm]

I've got a '76 TR7 and I've just replaced the rear wheel cylinders on both sides. I was trying to bleed the system, but although pedal pressure improved when my assistant was pumping, I never got any air or more than a couple of drops of old brake fluid returned. Also, when I've done this in the past, as you open the system, the pedal falls towards the floor. I went and purchased a vacuum type system and again, got nothing back and the vacuum level never dropped when opening the bleed screw. I'm a little confounded and don't know what to check next. Should I unfasten the lines front and rear and try to blow the lines out with air? I also think I have power brakes on this car... Is it necessary to have the car running to bleed them if they are power assist brakes? Sorry for the stupid questions, but I swear I've done this before on other cars with no problems. Any help is greatly appreciated folks. Thanks.

 

[tdskip]

Are you sure the bleed screw is all the way open?

If you disconnect the hard line at the rear cylinders do you get flow?


Hang in there.

 

[DNK]

Check that the thingy the the booster is attached to has a thingy in it that has to be removed.
Sorry for the weird answer . I can' remember any of the names.

 

[brujahstorm]

If I disconnect the other line (that is attached to the same wheel cylinder) while I've got vacuum applied, I lose the vacuum. Not sure about the "thingy" that needs to be removed. Do you have to remove something on the master cylinder side in order to bleed the brakes? Anyone else know what the poster is talking about? Note: I started on the driver rear because there is no bleed screw at passenger rear..rather, that line is attached to driver's side rear.

 

[TR3driver]

The service manual talks about removing the pressure failure switch from the underside of the master cylinder. Perhaps that is what Don is talking about?

There is also a pressure reducing valve (aka proportioning valve) for the rear brakes, but I don't know any reason it should be removed to bleed.

 

[DNK]

Gawd, thanks Randall. That's it
It's the switch. This was just talked about here or the TR7 site

 

[brujahstorm]

I see the wires under the master cylinder and assume this is the switch talked about. Does anyone know why this needs to be removed to be able to bleed the brakes? I'll try tomorrow and see what happens, but would like to understand how the system works. Don't know if this is mentioned at all in my Bentley manual for the car, but I'll check. I didn't consult the manual because I've bled brakes on other cars with this same procedure and no issues...

 

[DNK]

Brujah. You got the manual for that door stop?

 

[TR3driver]

If not, I've got a PDF I found on the web somewhere. Maybe it was on vitessesteve's site? Don't recall. Anyway, if you need it, I can put it up on Google Docs for you.

It doesn't really talk about why the switch has to be removed, but I seem to vaguely recall that the switch could limit the motion of the piston for the rear brakes and make it difficult to bleed them.

 

[DNK]

I can just send him the Doc

 

[brujahstorm]

I have a Robert Bentley manual for it someplace... I don't know how detailed it is, but I was told long ago that these are the most comprehensive. That being said, I would gladly accept and and all manuals or info you chaps have to offer My direct email is brujahstorm@gmail.com

 

[DNK]

That's the one.
I'll send you another for your computer.
It's on its way

 

[brujahstorm]

Found it in the book!! It talks about removing the pressure failure switch. Also, it says to bleed the front disc brakes FIRST, beginning with the passenger side, then the single rear nipple on the driver's side. I was always taught to go farthest to nearest the master cylinder. In any case, I'm going to try this method and see what happens, probably tomorrow afternoon. I don't understand what the switch does except maybe send an indication to a dummy light on the dash or something. I don't know why that should interfere with bleeding... If anyone knows, please enlighten me if you can. Thanks..

 

[DNK]

The switch tells you if there is a failure in the system.

(I think)

 

[brujahstorm]

Any idea why it would interfere with bleeding? There's not some sort of re-directing valve or anything in there, right? Looks just like a little switch. I don't understand how it could keep brake fluid from traveling to the rear brakes...

 

[DNK]

basically what is going on is it cut off the rear bakes lines ,
If I could guess.
There are a couple of east coast boys who will chime in later and answer that

 

[brujahstorm]

Thanks so much for the manual Don, that was very kind of you. Procedures are quite similar to the Robert Bentley manual I have, but yours has a great and detailed explanation of how the system functions...very nice touch. I'm assuming that the low pressure state triggered a fault and bypassed the rear brakes somehow as this is what the system is meant to do. By restoring proper brake pressure through bleeding, hopefully the system will work as it should. Anyway, hoping to give it a go tomorrow after work. Cheers.