Hope everyone had a great Memorial Day weekend!

A couple weeks back, while I was doing some motor work, I looked under the car and saw that there was brake fluid on the inner face of the tire - good thing i caught that before it was a worse problem while driving!

Car is a 1969, with manual(standard brakes), so no power brakes.

So I decided to replace the calipers, hoses, and machine the rotors. While starting to bleed the front brakes, I made the stupid mistake of answering a call from a buddy. I looked into the master, and the front reservoir had gone down to the orifice at the bottom. I though i caught it before it went dry, but maybe did not.

I decide to power bleed with the Motive unit I had, passenger rear(inside bleeder, then outside), driver's rear, front passenger, and then front driver's. all in all, After it looked like no more bubbles were coming out, I did a full bleed 3 more times at the calipers. The pedal was rock hard and the system seemed to be properly bled.

When I took the car for a test run, the pedal was very firm, but the car did not stop well, and seemed to require much more pedal pressure than before. I understand that usually new brakes will need some time to bed-in, but this was a significant difference from before - worse than it was with brake fluid soaked pads on the passenger side. When I stood on the brakes hard, the rears started to lock up, so it was obvious that the car was mostly stopping with the rear brakes.

I have read in a couple of older posts here, that if there was an issue with losing pressure in either the front or rear circuit, the distribution block(often referred to as the proportioning valve) has a piston that will shift to one side, and will not allow pressure to go to the "damaged" or "affected" circuit. At no point did the BRAKE light come on in the dash, while bleeding or driving, as some have suggested would be the case if the piston moved to one side of the distribution block.

Here are my questions:

1) By running the master cylinder low, did I mess that up? Do I have to bench bleed the Master? Did the power bleeding not correct any air in the Master? Remember, the pedal is very firm.
2) Is my main problem that the piston in the distribution block has shifted to one end? If so, how can I reset it, without removing? Do i need to force compressed air into it from the affected circuit(not desirable, as I am concerned about spewing fluid all over the car)? Do I need to replace it?
3) Have I caused another problem here, that I have not described?

What can I do to correct this problem???

I have read and re-read a post from TOM454 from around 2010, which explains the bleeding procedure in great detail, but is not clear about what I may have done to cause this issue, and preventing me from properly bleeding them?

Thanks in advance everybody,

Rob

 

I replace all hoses/rebuilt calipers and master cylinder. but did NOT bench bleed the master.....bad mistake!. The brakes never seemed to stop the car properly. After a couple days of fighting the weak brakes, a member convinced me to benchbleed the master cylinder and clean the distribution block-piston assembly...then re-bleed the entire system. That solved my problem.

Do NOT use compressed air The piston could be "gummed up" so that it cannot return to the center-it may need cleaning or new O-rings.

I do not think you damaged anything. The distribution piston COULD be off-center inside the housing without the brake warning light coming on. If you remove the sending unit from the bottom of the block, using a small mirror you might be ble to recenter the pistion with a thin screwdriver.







Orifices need to be pushed/tapped out of the block with a thin metal rod from the opposite end...…..






Piston assembly with springs and orifices on each end of block...…..

 

I've run several master cylinders in a variety of vehicles low in the process of bleeding, over the years, but never needed to rebench-bleed one (just bleed the air out of the system). And that your car had a firm pedal points to something else.

On the pads I would either replace the oil-soaked pads, or, you might try putting 1/8" of solvent in a pan and put them in face-down for 5-10 minutes or so. The solvent will draw the brake fluid out. Then, after they dry out, run some 80 grit over the surface (with a good dust mask on of course).


.

 

you say you replaced calipers , hoses and machined rotors...…...I'm assuming you also put on pads but forgot to say that. So if that assumption is correct......Have you had someone push the pedal and see if you can still spin the front wheels ? Certainly sounds like no air is in the system seeing how you have a firm pedal.

 

Assuming everything is new up front...
You can remove the master cylinder and bench bleed it - pretty cheap and easy to do. Then hook everything back up and gravity bleed the rears first, then the front circuit. That's what I did on my 69 after I replaced the MC to prop valve circuit lines with
SS lines. I had to remove the MC to get at the forward lighting and engine harnesses which I replaced. I don't think you did any damage, but you have to ensure the MC has been properly bled if it goes dry. As a bonus, you can paint the MC while it's out of the car and make it look
like new!

 

Hi Mark:

Thanks for the reply - yes, I should have been clearer...I did replace the pads with new semi-metallic pads. Did not know you could try and draw the fluid out, and they were reasonably enough priced. Might as well start all new.

 

Thanks Doorgunner! yes, I'll try to remove and disassemble the block and see if I can centre the piston.

jpatrick62 - i'll also remove the master and bench bleed it. Supposed to be a rainy weekend here, so it should give me some time to do it.

On a related note, the engine work I did was brilliant. The car has run the best, since I bought it 3 1/2 years ago...pulls very hard, .just doesn't stop well!!

 

I had to disassemble the block on mine as well, it was stuck to one side blocking flow to rear, and no light on.