MGB Brake frustration

[Bob Claffie]

Recently bought a 74.5 roadster. Drove it home 200 miles and the brakes were "marginal". Low, soft pedal, car always slowed down and stopped but without a great feeling of confidence. Pedal felt like there was an internal leak in the MC. Replaced the MC, no improvement, exactly the same pedal response.

After several bleeding sessions, , including backing off the distribution block on the inner fender panel I finally got a high, hard pedal, just what I wanted. Drove the car about five miles and we were right back where we started. About a half, soft pedal.

As far as I can tell the calipers are good and the rear wheel cylinders are OK, too. No fluid leaks either. new shoes and pads all around and the rotors look new also.

I hate to spend the $$$ for professional help but I am just about out of ideas. Who can give me the clue to fix this ? Bob

 

[Rut]

Bob,
My first thing to check would be the front brake hoses...if they are old they can allow fluid to go to the caliper and not fully return. I've had a soft pedal due to this as well as excess friction on the front brakes. Drive around a little and see if the front brakes get hot...if they do, then replace the lines.
Rut

 

[Brinkerhoff]

Its almost hard to realize that the last chrome bumper MGB is 40 yrs. old isn't it? Time takes its toll on everything and once you open a braking system , you'll be replacing everything if you're wise. Even though parts don't look bad , they are still way past their prime.

 

[Bob Claffie]

Rut the previous owner replaced the 3 flexible brake hoses with the braided covered ones. Probably in the last few years. One less thing to change.
Brinkerhoff the previous owner apparently had the same problem as I am currently having. The vast majority of the braking system is relatively new, I am not dealing with original parts. Replacing EVERYTHING seems over extreme to me. Maybe in another couple months i will throw in the towel and do it if all else fails

 

[rick_ingram]

Rut the previous owner replaced the 3 flexible brake hoses with the braided covered ones. Probably in the last few years. One less thing to change.
Brinkerhoff the previous owner apparently had the same problem as I am currently having. The vast majority of the braking system is relatively new, I am not dealing with original parts. Replacing EVERYTHING seems over extreme to me. Maybe in another couple months i will throw in the towel and do it if all else fails

Braided hoses can still deteriorate. They swell inside...the braiding hides any external deterioration.

 

[Rut]

Bob,
Although the MC is new, is it adjusted correctly? You may have a situation where the seal does not completely pass the hole from the reservoir and you get incomplete fill. Check this adjustment and also check the pedal return spring. This is a shot in the dark, but the cost is $0.
Rut

 

[DrEntropy]

Another thought is the rod in the MC. Check for any oval shape in the hole the pin goes thru and any wear on that pin. I've pulled 'em apart and found pins that looked like a crankshaft, with rods having the hole wallowed out. That will give you about half the stroke needed to make the MC work properly.

 

[Nunyas]

The only thing that I can think to add, and hasn't been mentioned already is rear brake adjustment. The rear brakes are manually adjusted. If they're not adjusted properly, you'll have all that extra travel to make up before the rear shoes begin to bite and build up enough pressure to bite at all 4 corners. If the travel is excessive, you could start leaking out the rear cylinders.

 

[Brinkerhoff]

I agree with Nunyas, the brakes are bled correctly with the rear wheels locked with the adjusters, then turned back one click at a time until the drum rotates freely while just catching the drum.

 

[Brinkerhoff]

.. with the shoes just catching the drum .

 

[Bob Claffie]

Never knew the rear shoes should be locked up before attempting to bleed. i will add that fact to my bleeding procedure. It can only help. Thanks, Bob

 

[Bob Claffie]

This is an interim report followed by a simple question. Due to the lack of a bleeding partner it's taken a week or so to get back to this project. Today was THE DAY. With the rear brakes fully tightened and the center fitting on the brake failure switch backed out a friend and I proceeded with the bleeding exercise.

We started with the LF and going around the car clockwise and ended with the LR. As usual both front calipers bled only fluid, no air. The RR had some air, but pushed out fluid after the third or fourth iteration of "pump,pump,hold". So far so good. The LR was full of air and it took multiple sequences to achieve only fluid coming from the bleeder.

At this point I have a good, firm, high pedal and I am afraid to test the car. Once previously I reached this point only to have the pedal sink to 50% after a few miles.

The question I have is: is it possible for somewhere in the braking circuit to have a "leak" so small that NO FLUID is lost but there is air entering the system ?????? Sometime in the next week , or so, I will force myself to take it out on the local roads and keep my fingers crossed that the brake pedal remains fully functionally. If it fails again I will be out of techniques and ideas and may have to get professional help.

 

[JPSmit]

If it fails again I will be out of techniques and ideas and may have to get professional help.

For you? or the car?

I am very hopeful that this will work for you. I know that air can sometimes get past bleed screw threads even as they are being bled - I have started using thread sealer (for plumbing)

Not sure enough about B Master Cylinders to know if you can get an air bubble in the MC itself?

What I do know from repeated experience is that just because a part is new does not mean it can be relied upon. Last year a very large chunk of my summer was given over to working on my Clutch hydraulic system - in the midst of everything I realized the ends on the clutch line were not sealing properly even though the part was brand new - I finally had to make one using my old ends. Don't assume your hoses are good, or anything else for that matter. If nothing else, many of these parts are being jobbed out to factories in a part of the world not known for precision work - and moulds are old and and and. increasingly I am finding tolerances are not reliable - or mixed metric and imperial.

 

[Mickey Richaud]

Glad you got it done, Bob, but one observation that may have contributed to the difficulty: The common order of bleeding is to do the farthest from the master cylinder first and work your way to the closest. Any air trapped in the system is going to take the path of least resistance and may travel to another line (likely the one you've just bled!) rather than out the bleeder. If you do need to bleed them again, try this sequence.

And make sure it's not the master cylinder itself that's at issue. If there's air in there, you'll never get it right.

Good luck, and keep us posted.
Mickey

 

[Brinkerhoff]

To answer your question whether air can enter the system without showing a leak ? I don't think so- the system is either sealed or not . I think you are on the correct track but as Mickey said ; the typical procedure is to start the furthest from the master cylinder. Just turn the rear adjusters back a click at a time just until the drums turns by hand.

 

[Sarastro]

"the third or fourth iteration of "pump,pump,hold"..."

Hmmm. Not sure I understand this, but it might be the problem. When bleeding the brakes, you open the bleeder, push the pedal to the floor, close the bleeder, let up on the pedal. If you let up on the pedal without closing the bleeder, you can suck air (or fluid with bubbles in it) back into the system.

That said, I doubt your problem is bleeding. If so, you would never get a hard pedal. It sounds more like a fluid-return problem, which could be a bad hose or something else in the hydraulics. The pedal adjustment at the master cylinder can be critical, and if it's off, can cause this kind of problem. (I'd expect you'd never have a hard pedal, if that were the problem--but still, it's well worth checking.)

 

[Bob Claffie]

It's time for a "true confession". I am basically a cheap person. While reassembling the new MC into the car I REUSED the copper sealing washers . Did I shoot myself in the foot trying to save two bucks ? I apologize to all who have offered good advice which may have had no chance of helping me fix a self induced problem.

 

[JPSmit]

It's time for a "true confession". I am basically a cheap person. While reassembling the new MC into the car I REUSED the copper sealing washers . Did I shoot myself in the foot trying to save two bucks ? I apologize to all who have offered good advice which may have had no chance of helping me fix a self induced problem.

I doubt very much that is the problem (as much as confess IS good for the soul :grin: ) that said, they are easy to re-anneal - heat the washers to red hot and air cool and you are good to go.

 

[DrEntropy]

If there's no evidence of leaking at the banjo fitting, there's no problem with reusing the copper washers.

 

[Bob Claffie]

OK, in a nutshell, this is what is wrong with the MG brake system. Once the system is properly bled and the pedal is high and firm, After driving LESS THAN THREE MILES the rear brakes are full of air again and the pedal is soft and low (goes about two thirds of the way down. I'm fed up. Taking offers on a very lightly rusted body with OD and new exhaust & tires.