I have a 69 400 car with power disc brakes .ive bled brakes a million times but i am seeming to have trouble with getting this system to bleed the air out.at first i thought i had a restriction in the lines so I air checked all the lines so i thought it was my master cylinder.so i got another master bench bled it and put it on and still am having troubles both at the front and rear corners.there are no restrictions at the metering valve,proportionig valve the rear metering block,line lock (aftermarket),or lines but it still will not gravity bleed at all.Tell me im missing something obvious i talk to my co-workers at the dealership and they wanted me to check all the things that ive already checked.im almost to the point of insanity.anyone have a pearl of wisdom or a similar experience with these early disc brake systems.i'd appreciate some info. [img]http://J:\DCIM\101_SONY[/img]

Did you block the metering valve when bleeding the rear brakes? Take off the dust cap and wedge a big screw driver or similar betwwen the valve and brake booster before you start pumping away on the pedal.

Gravity bleed??? I only get lucky enough to get them to gravity bleed about half the time. Especially with a 'line lock'. You need to force bleed them. Either with the 'pedal' technique or the 'vacuum pump' technique.

I use a cheap vacuum pump on my disc brake system. It has worked every time.

If you have repeated loss of system pressure, look for slight leaks at the rear wheel cylinder line fittings, the rubber boots, or at the bleeder screws. In the front, check the caliper piston seals and bleeders for any sign of fluid.

Have you had the calipers off, and if you did is it even possible to get them mixed up? If they are reversed, then the bleeder valves may be at the bottom with the air at the top. Aside from that, it seems that the rest of the posts cover
everything else. Although you've done it a number of times, I will assume/suggest starting to bleed them with the right rear, then left rear, on to right and then left front, in that order. If you still consider or want to rule out the master cylinder, you could take the lines loose from it and use plugs to seal the master cylinder off and make sure the master isn't bleeding off.

I had the same problem when I did my disc brake changeover. I was bleeding my system more than the Black Knight in Monty Python. Turns out the idiot that installed the calipers had them reversed.
Oh, wait, that was me......

tried vacuum bleeding the brake system.tried to hold open the valve mounted on the left of the master cylinger(via prying it against the booster)i could not get it to move.does this mean that this valve is bad?I have never in my life had such a problem with bleeding a brake system.I feel like this my first day working on cars.I feel like a rookie.

Jeremy

If the valve is out and will not return you either have a bad proportional(bleed down) valve or an obstruction. At this point I would pull out the bleeder screws and pump the brakes and see if fluid comes out all four corners. You need a good 'flush'.

I dont think i need to flush the system being that all the component except for the metering valve are new,and i am also using fresh fluid. but i will remove the valve tonight to see if i can unstick it. if i cant, i will make a call to year one and order a new metering valve.thanks everyone!

Jeremy

i was able to force the valve open on the bench but now it wont return back.my question now is what would happen if I ran this valve like this(open all the time).In theory it would always allow brake fluid through right? it would just mean that its purpose as momentary "hold off" valve would not be there anymore.but would that be enough to cause any adverse brake problems.I probably shouldnt take any chances on something involving safety like the brake system.I am just wondering how important this valve is.I would like it on the car as far as appearance in the fact i would like to maintain the brake system as looking correct.i dont know,i'll probably just end up getting a new valve.


Jeremy

Without that delay valve when you press the brake pedal your front disc brakes will engage before your drum brakes apply, if they ever apply. That would place your braking bias on the front brakes. In wet conditions and in high speed braking you may have a serious disadvantage over properly metered four wheel braking.

It's easy to check the delay valve on-car; when the brakes are applied, the little button on the back moves out. If yours would not move in, are you sure you did not have a residual valve or check valve in your line lock or in your master cylinder to allow brake fluid to return freely up the line?

I think you only have to depress that button for bleeding when you are using a pressure bleeder at the master cylinder or the pump-the-pedal method, not with a vacuum or gravity bleed. I did not have to depress it for a successful vacuum bleed. The first time I bled them after replacing the lines and master cylinder, I bench-bled the master cylinder and filled and capped it, then used the vacuum bleeder to fill all the lines from a bottle, then attached the lines to the master cylinder. This dramatically reduced the number of trips from the underside to the engine compartment to refill the master cylinder. Final bleeding was very quick and easy.

Jim's suggestion for opening all four bleeders and seeing if you get a squirt from each corner is valid. If you have a kinked hose or line or other obstruction you can bleed and bleed and never overcome it. Fluid should squirt freely from each bleeder screw with a single press of the brake pedal. Unless you have a helper to close the screws while the pedal is still down you will need to bleed again.