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MG MGA - Soft Initial Brake Pedal

For those who followed my recent front MGB brake caliper saga. You will recall I discovered the brake pistons were jamming in the pots because the seals had been damaged during assembly by the supplier. The braking issues had gone on for almost 10 years and I had accepted the feel as normal! I am pleased to report that the new calipers are working just fine and the car is running very freely with no brake dragging due to the previous slow release.

However, now to the title of this thread, I notice that after the car has been unused for a week or so, the initial brake application is somewhat soft. A couple of applications and it is back to very firm braking feel and working brilliantly. I am just wondering why this initial softness should be? Is it because of the extended lay offs the pistons; the rear drum linings; and the MC piston have backed off and it takes a couple of presses to get it all back to operating settings? Three things worth saying: the clutch is perfect; there is definitely no air in the system; and there are no leaks.

Steve
Steve Gyles

Steve,

I have the same experience. I cannot say for sure that the symptoms are the result of my conversion from front drums to MGA disk brakes. The front drums were a basket case.

After a week or so of not driving her it will take one pump to bring the pedal back to its normal, firm state.

John
John Backman

Steve,
This is most probably a result of your new seals with plenty of flexibility, see http://www.mgaguru.com/mgtech/brakes/bt104.htm
You could also check your rear brakes' adjustment - there is an issue with the adjusters running out of travel and needing a spacer silver soldered to their base so you can get maximum adjustment.
Mike
Mike Ellsmore

Mike

Thanks for the link. That could certainly account for it. As I said, I never had a chance to feel what the brakes should have been like when I converted to discs because they were faulty and I did not realise it. So what I am now experiencing is probably the norm. When I fitted the new calipers I also changed the 3 flexible lines, adjusted the back brakes and bled the entire system very thoroughly.

steve
Steve Gyles

Mike, what was kind of interesting to me, is I had the opposite problem with my front brakes when I assembled them a few months ago. with new cylinders, shoes, and adjusters, the drums (which I had lightly turned) could barely be installed, and they had a lot of turning resistance. I had to remove a little metal from each of the shoes with a die grinder.

Personally, even if the adjustment turns out to be good, I would still bleed the brakes again as the next step, even if you are "sure" you got all of the air out the first time.

-Del
D Rawlins

Del, the issue with the brake adjusters needing a spacer only relates to the rear brakes (one adjuster).
Steve, it would be interesting to check the pedal pressure every day for a week (while parked up in the garage). Measure the depth of the first pedal press from the floor.
Mike
Mike Ellsmore

Yeah, I'm aware of that. I just thought it was interesting that so many have wound up with loose shoes, while mine were too tight, even after having had the drums turned. I had no issues adjusting in the rear, either, no shims required. Though I didn't have to remove any metal from those. I can't remember if these were shoes I bought, or ones my dad bought for the car back in the '80s.

-Del
D Rawlins

Mike

When I get a spare moment I am going to check the system through again, including another full bleed. Finding time at the moment is not easy as I am a full time carer while the wife recovers from her hip replacement. I guess one does not always appreciate how much time and energy is taken up looking after the essentials of house management plus a temporary disabled person!

I just did a pedal measurement after 3 days of non use and I am not that happy. First push was an excessive 2" of travel - half way to the floor. Subsequent pushes halved that. As I said, when in operation, the pedal feel is very firm; no spongy feel as if there is air in the lines.

Steve
Steve Gyles

Steve, Seems a bit much travel - would be worth another bleed. I like the gunson easi bleeder as a way of getting all the air out
http://www.gunson.co.uk/item.aspx?item=1818
(You do have two bleeder screws on each calliper a the highest point and bleed the inside bleeder first?)
Mike
Mike Ellsmore

Mike

That's what I use. Only one bleed nipple per MGB caliper.

Steve
Steve Gyles

Further to Mike's query about 2 bleed screws. It got me thinking about the single bleed screw at the top and on one side of the MGB caliper. How can you be sure that the small drilling through to the piston on the other side has filled with fluid? Will sealing up the MC to prevent drainage, then undoing the bleed screw and allow to stand be sufficient? My logic says that under normal bleeding (under pressure from Eezi Bleed) that small passage way has very little chance of ridding itself of air.

Steve
Steve Gyles

Steve,
I'd remove the calliper bolts and orient the calliper on the flex line so as to move the air.

Regards
Colin
Colin Manley


What type if brake fluid are you using? Silicon (DOT 5 in the US). Is what I use in all my older cars. I am very happy with it (but let's not start the DOT 3 vs DOT 5 discussion). Silicon brake fluid is very difficult to bleed. Air stays suspended in the fluid in much smaller bubbles for a much longer time. It is best to bleed them once, let the car sit for a couple of days and then bleed again. Some times it takes three bleeds to get all the air out. Just a thought.
F. J. Bruns

Well that was most interesting. I just bled the entire car again in the standard Eezi Bleed manner. I got about 6 pin head bubbles out of the left caliper, but really nothing of significance. All the others were crystal clear. With clear piping still attached to the caliper nipple I then put a G cramp on the right hand caliper to squeeze the outboard pad against the piston, pushing it back into the cylinder, at the same time releasing the nipple. 10 large bubbles came out! It would seem that getting that initial fluid down the oilway to the outboard piston is not guaranteed with normal bleeding?

Brakes lovely now! Yes silicon for 19 years. Great stuff. Won't listen to a word against it. Lot's of old wives tales about it. All wrong.

Steve
Steve Gyles

This thread was discussed between 16/07/2015 and 20/07/2015

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This thread is from the archive. The Live MG MGA BBS is active now.