Old Car Brake Bleeding Problem

[Hugh R]

I have an old car that I kinked a brake line in the engine compartment pulling the engine for a re-build. Replaced the line, and the pedal goes to the floor after repeated bleeding. The system was fine before replacing the kinked brake line.

Here's what I've got, single braking system, lose one brake, lose them all. Master cylinder is under the pedal with a remote reservoir on the fire wall. It gravity feeds makeup brake fluid to the m/c. From the m/c the pressure line goes across the engine compartment and up to the vacuum booster. I can bleed the m/c and booster, get the air out, and the pedal gets hard.

However, when I connect from the booster down to the brake distribution block is when the pedal pushes to the floor, no matter how much I bleed. I made up a pressure bleeder to use at the reservoir and I initially pushed lots of air out, went around five or six times and no more air.

The distribution block doesn't have a proportioning valve, just one threaded hole for the brake light pressure switch, and three other threaded holes one to the front left brake, one to the front right brake and one to the rear wheels which splits to the rear brakes on top of the differential.

No leaks that I can see. several people told me its my m/c and that fluid must be bypassing around the cup seal. I don't see how this could be the problem cause I don't get leaks or additional air coming out of the lines, and the pedal gets hard when I bleed just the m/c and booster.

I'm going to get some fittings and block off the rear brakes, and then the fronts to isolate the distribution block and if the pedal gets hard, start adding lines back to the front and then rear brakes. BTW, the car has four wheel disks. Any other suggestions, this is driving me nuts. Oh, and its British trash to boot.

[RoninLB]

can you pump up the hardness by pumping the pedal? If so and you leave psi on the pedal does it go to the floor?

old cup seals often get damaged when pedal bleeding.

[Joeaksa]

Hugh,

One problem with the old Brit cars. Their pedal goes down a certain amount then pressure keeps it from going futher. When you take a line off or anything to relieve pressure, it allows the piston to go futher.

If the master cylinder has been on there a while it can build up a ridge in it where the pedal travel stops. If you for any reason can push the pedal futher than normal, I have seen times where the rubber sealing ring can be torn or damaged to the point where it no longer will build up pressure.

Might take a line directly off of the MC and confirm that you have good hard pressure before moving any futher.

[Hugh R]

Thanks guys, forgot to add that I suspected the m/c for exactly the reasons you suggest about the lip or the corroded area of the bore where the piston and seal rarely travel. I pulled the m/c and honed and put in a new set of seals. It looked fine when I pulled it, nice and smooth and shiny bore. Can't pump it up as you say, just goes smoothly with some resitance to the floor each him. As I said, I bleed the m/c alone (put a bleeder on the pressure line side instead of a brake line, and bleed it and got a hard pedal. Did the same with the next item in line, the booster. Pedal went to the floor, as now, until I burped the last bubble out, then hard pedal. Gettiing at the distribution block is a bit(h with the engine in. I'm so stupid for not dealing with this when the engine was out. It never occurred to me that I would have such a problem. I still haven't driven the thing. But its timed, and the SU's balanced and it runs well. Oh! and I ran it up to about 3,000 rpm and no vibration off of the engine or trannie, which is the reason I pulled the engine in the first place.

[competentone]

Maybe I'm not understanding your description or the system, but have you bled the system at each brake caliper?

[notfarnow]

Originally posted by Hugh R
Oh, and its British trash to boot.

Oh well if it's just some old british junk, I'll take it off your hands for a few hundred bucks

Do the brakes need to be bled in a certain sequence? Some cars are fussy about which lines get bled first, otherwise you end up with air bubbles trapped in the lines.

[Joeaksa]

Then might take the M/C off and fill it with fluid, then bleed it, tapping the cylinder with a wrench to get bubbles out, and install it and work backwards from there.

[Hugh R]

Yeah, bleed from the brake farthest from the m/c and worked my way forward. The m/c is a little slow coming up when I depress it, but I don't think that's the problem.n On the other hand, I'll replace it in a minute if it'll solve the problem. Again, the the m/c and the booster get solid real fast once I get the air out of them. Its downstream where it gets fuzzy. That tells me the m/c is OK.

[Hugh R]

Originally posted by Joeaksa
Then might take the M/C off and fill it with fluid, then bleed it, tapping the cylinder with a wrench to get bubbles out, and install it and work backwards from there.

Been there, done that. The m/c bleed got the pedal solid, as did doing thing next thing in line which was the vacuum booster.

[Joeaksa]

Then tap the line, as well as anything connected to it to get the air bubbles to release and come out.

[Steve PH]

The problem might well be the master cylinder but old vacume brake boosters can leak internally leaving you with no pedal pressure (I nearly crashed an old MGB GT when its remote vacume booster failed, so I do speak from experience on this!), try bi-passing it and see what happens?

[Rot 911]

Hugh my Bugeye Sprite had the same set up and I had the same problem you did. At least in my situation it turned out that air was trapped in the distribution block. I ended up having to crack the 3 lines at the block one at a time and get the air out. Also made a mess with brake fluid seeping out.

[Hugh R]

I'll try bypassing the boost and cracking the distribution block lines and I'll let you know what I find. Thanks guys.

[Hugh R]

Update:

I pulled the m/c and honed once again and cleaned and got it working better, it was sticking at the bottom of the bore. I bypassed the vacuum boost and am now getting brakes pressure! I'm going to test drive the car up and down my private street next week and feel out the recent engine and trannie rebuild (I haven't driven it yet) and then reconnect the booster and and see if its the problem.