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MG MGA - Excessive Brake Pedal Travel
Now that I am in year two of MGA ownership I would like to figure out why my brake pedal is not as firm as I think it should be. Last year I replaced the shoes on all four wheels of my 1500. New wheel cylinders, new rubber brake lines, rebuilt master cylinder and clutch slave. The brakes were bleed using the EZ bleed. I have the brake adjusters set properly (one click back after locking the wheel). I have the nut on the master cylinder push rod adjusted correctly.
Despite all the work I have never been able to get a real positive feel from the pedal. The only time it feels right is the first use of the brake after I release the emergency brake.
The last thing I can think of is replacing the drums but I hate to spend money on a "fix" that might not work.
|I would bleed the brakes again using the old fashoned two person system. Open bleeder, have helper push pedal down, close bleeder, release pedal. Repete as necessary. When you think you are finished, with the bleeder closed, have your helper push hard on the pedal, then open the bleeder, the high flow of fluid you get will carry out the last little bubbles.|
It may also be the shape of the brake shoes, years ago the parts house machine shops would arch the shoes to match the drums to get full contact. No one does this any more so you may need to wait for the shoes to wear to match the drum, this will also firm up the pedal.
|Hi Jeff, After I had replaced my rear brake cylinders and rerubbered the master , I had to bleed the system at least 3 times. Each time after I had driven the car a couple of miles . The motion of the car seemed to shake free any air bubbles trapped in the system - I also bled the cutch at the same time, finding air bubbles in there too - I used the Eezi bleed system too. cheers Cam|
|I knew you guys were going to say that I needed to do a better job bleeding. The truth is I used an entire gallon of brake fluid to bleed both the brakes and the clutch. I spent much time using the two man method. I finished with the EZ bleed and really let it run. I'll try again but I suspect its something else. The shape of the brake shoes is an interesting point but I did put 2500 miles on the car with no noticeable difference.|
Any ideas why the first push of the brake after releasing the emergency brake makes a difference?
|What happens if you "pump" the pedal?|
|M F Anderson|
You use the phrase "positive feel from the pedal" but your title suggests you are getting more travel than you would like. You indicate that all adjustments in linkage are correct so apparently you have the correct free play in the pedal. How far down does the pedal go? Does if feel "mushy" or firm at the end or travel? Does the car stop in a straight line without pulling? Have you done a "lock up" test on various road surfaces? People who have only ever driven cars with power brakes sometimes find the unassisted MGA to feel peculiar.
Regarding the different feel after setting the parking brake..this would suggest that the shoes are repositioning themselves when the parking brake is used which could be caused by out of round drums. Have you measured the drums for diameter and trueness?
Just some thoughts.
|G T Foster|
|Jeff, a mechanic once told me that more that half of the problems he found with excessive pedal travel were due to poor rear brake adjustment. Make sure that the rear brakes are fully adjusted onto the rear drums: get the shoes just binding and then go back one notch.|
|J H Cole|
|Thanks for the input.|
Pumping the pedal raises the pedal to some degree. The stroke to engage the brake is excessive - I think by anyones measure. It does firm up to a degree at the end of the stroke and it does brake in a straight line. I can lock them (or at least one) up. By the end of the stroke I have the pedal fully engaged which does not make me feel all that safe.
Regarding the drums, I'm not sure what the tolerances are so I have not measured them all. I did take a diameter measurement of one and if I recall correctly it was 12.5 inches.
I was told by John Twist after he rebuilt my m/c that the pedal in a properly working MGA should be firm almost from the very top of the stroke.
I should probably have one of the guys in our MG club drive my car for an opinion. Maybe I'm worrying about nothing. I mean what the heck...it's just the brakes....what could happen? Ha!
Based on all that you have said I would try to test the MC. If you disconnect the line from the MC to the 4-way union and dead-end the line you could check to see if the MC is indeed holding pressure. This is a bit painful, I know. I've done this sort of thing on the bench quite successfully. An old wheel cylinder and flex line can be helpful to adapt and bleed the line. An old section of brake line can sometimes be modified. Yah, I know its a pain, but I think you need to eliminate the MC as the culprit. It would not be the first time a rebuilt MC was bad. I would also take a look at the new rubber hoses. This is a shot in the dark, but could they be faulty and actually expanding slightly? There are a lot of crap parts out there although I've never actually heard of this. I did my brakes a couple years ago and the pedal is very firm and doesn't move very far. I do have front discs. I'm curious to see how your concern is resolved.
|G T Foster|
|Generally if putting the emergency brake on improves the feel of the brake pedal it demonstrates that the rear brakes require adjusting.|
If the brakes improve by "pumping" the pedal then it can either be air or poorly adjusted brakes.
From a personal point of view I have found it impossible to lock and release the brakes by one notch only. My brakes on my MGA are very firm although I do have a 1600 with front disks.
I personally would adjust all the brakes up as tight as possible first.
|Robert (Bob) Midget Turbo|
|Points to rear brake shoes not adjusted close enough. Did you skim the drums?|
I have also heard of a case where the wall of a brake, or clutch, rubber hose had failed so that when you applied pedal pressure it balooned the hose.
Have you tried driving a friend's MGA to see if his/her brake pedal feels different?
Let us know what you determine.
|I'm wondering if the non return valve in the master cylinder is not working or is in the clutch side by mistake. It won't be the first time that has happened.|
|I think Malcom has it nailed, a problem with the non return or residual pressure valve in the master cylinder. That's there to hold a few psi of pressure on the brakes to prevent the springs from retracting the shoes all the way and cause a long pedal travel. Might be in the clutch side, but more than likely just not there or stuck open.
|Wrong function for the "non-return" valve in the master cylinder. It does not hols the shoes out, it only slows the return of fluid so you an pump up the brakes if you have a low pedal situation. The fact that these brakes will pump up implies that the non-return valve is functioning properly.|
The MGA master cylinder is 7/8-inch bore. Occasionally a 3/4-inch bore unit from a 1098 Spridget may find its way into an MGA. The small bore MC requires almost full stroke of the brake pedal, right to the carpet for full actuation. There is a "7/8" or "3/4" cast on the side of the MC body, but you have to dismount it to see. It must be easier to remove the end plate to measure the bore.
|The part about it feeling right after using the parking brake makes me think it is a problem with the radius of the shoes not matching the drums. If the drums are worn or surfaced oversize, their radius will be larger than that of the shoes and the center of the pad will contact the drum first. Then the shoe will flex until the ends of the pads touch the drum giving excessive travel to the pedal. In the old days when most cars had drum brakes most brake shops had a shoe arching machine to cut the shoes to match the radius of the drums. You should definately measure the drums to make sure they are not too worn, but the problem should improve as you drive and the shoes wear to match the drums. I had that problem with my 1500 when I did the brakes many years ago. I just adjusted the brakes until they dragged slightly and drove it until the shoes bedded in and then re-adjusted them and they worked perfectly.|
|That's a lot of information to sort through. Thank you. I should add that John Twist had me send the M/C back to him last year after I reported the problem. He went through it again and I'm confident that if anything was put together improperly that he made it right the second time. |
Bob - your right. It takes two turns of the adjusters to fully release the shoe.
Regarding my ability to pump up the brakes - one pump will bring them up just a tad, multiple pumps doesnt really do anything.
Regarding measuring the drums - what are the tolerances?
Barney - its a 7/8 bore. I had the damn thing out four times last winter - I know it well!
I'm going to go through everything again in the garage and then if all else fails I'm going to tighten the adjusters and drive it for a bit and see what happens. Just as soon as the ice melts and salt washes away! Uggggh.
|The workshop manual says the drums are 10" diameter, but I couldn't find any maximum oversize spec. The general guidance for most brake drums is that they can be turned to a maximum of .060 oversize.|
This thread was discussed between 03/02/2009 and 05/02/2009
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