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MG MGB Technical - Braking performance
|I've had 2 very mechanical friends drive my 73B roadster in the past 2 to 3 weeks. The first before replacing the master cylinder and the second after.|
Both these people said they felt the brakes were not working properly in terms of the amount of foot pressure it took to come to a stop.....i.e., way too much. I guess I'm used to driving it so haven't noticed this as a problem.
I did a quick/temp rolling lock up of the brakes going about 25 mph this afternoon. I did this a couple times and it seems that only the rear passenger side wheel is actually locking up and leaving a skid mark.....about 5' long. I didn't do a full stop lock up so that might produce a different result.
My friend this afternoon wondered if there might be some sort of balancing valve in the front/rear dual brake line system, or if maybe there was an issue with the wheel cylinders and/or calipers.
For typical stopping they work fine. The question becomes how they would act in an emergency.
|BH, There is no balancing valve in the MGB braking system. That said the bear brakes/brake shouldn't lock up before the front. Most of us drive cars with power brakes for daily drivers. With no servo/ for 74 and older, the MGB does require more pedal effort than we are used to using. Do you know the age and condition of your pads and brake shoes? The rotors and pads can become glazed with resultant decreased braking performance. When I suspect brake problems I like to jack all four wheels off the floor and check that the brakes aren't binding and the calipers not causing the front brakes to drag. I will then have a helper apply the brakes and try turning the wheels. I know that isn't very scientific but it usually tells me if I have a problem.|
From what you describe, your rear brakes need adjusting. With the braking forces unequally distributed, you've only got three brakes giving good braking, so it's no wonder that you have to apply a lot of pressure at the brake pedal! Also, your front brakes should lock up first, so I'd suggest bleeding them. Get all four working together in unison and you'll be surprised at the difference.
|I would vote for high temp racing pads being fitted or the pads having become glazed. At the back 15 mins twirling the adjusters should see you OK, if not then a quick bleed of the lazy cylinder should fix it. I had DS11s on my car for a while and it behaved exactly like this, on the road it would stop but needed a big shove and because the balance was wrong you felt like you had no brakes , on a track once they warmed up it was brilliant. I went over to Greenstuff and the car was transformed, I do not have the servo and prefer it that way, the car stops really well with well modulated and light , but not too light, pedal presure. I did also put stainless pistons in about 3 years ago and they have fixed the annoying brake drag problems these cars get if they are not used much.|
|Well, found a problem. The drivers side inner caliper pad was not in properly. Somehow it had become twisted so that part of the pad was hanging out over the edge outer of the rotor. Couldn't see why this happened as at first I thought it was just stuck while properly aligned. I'll take a close look during reassembly.|
I've taken the rotors in for turning (they haven't been turned down before) and will put on new front pads.
The back brakes look okay. Am hand sanding the drums and pads with 80 grit paper.
We'll give this a try and see if it makes a difference.
|It might just be worth checking that the correct cylinders are fitted in rear.Several sizes can be installed and the wrong ones could cause the problem you describe.|
|"Both these people said they felt the brakes were not working properly in terms of the amount of foot pressure it took to come to a stop"|
Compared to what? Are they experienced with MGBs or any other British cars of that era?
As long as you can lock the front wheels on dry tarmac with progressive pressure the brakes are fine.
A sticking piston one side *should* cause the car to pull to one side which *is* incorrect ... unless you have a sticking piston both sides!
Rear cylinders have relatively little effect on overall braking. If one rear is locking before the others this can indicate oil or hydraulic fluid contamination. Adjustment of the rears shouldn't have any effect on whether they lock or not - they shouldn't lock before the fronts at all. Apart from the aformentioned contamination, the only other thing that should cause the rears to lock at different times is a siezed wheel cylinder, or possibly slackening off one wheel when they have both bedded in over many miles.
|Paul Hunt 2|
|I fitted some of the aeroquip brake lines in my 67B and found the improvement fantastic. I immediately noticed a substantial reduction in effort required to achieve better braking with the new lines. There is a much faster response with a much better "feel" delivers through the brake pedal. This coupled with fresh fluid and a proper bleeding will help ou for sure.|
Can you give me a web link to the Aeroquip brake lines?
|Oh, I forgot to ad above. I replaced the rear brake cylinders and put in all new springs. The drums were fine and my local garage (trustworthy) agreed that they were in good, smooth and clean condition. I did not replace the brake shoes as I have a minor axle seal leak issue to deal with on both sides and will put the new shoes on after that is done. The existing brake shoes are actually in pretty good condition.|
After doing all this all 4 wheels are now locking up properly.
|Sorry for the delay - here is the website for the lines in the US. They are made by Goodridge and I can highly recommend them.|
All the best.
I take it the replacement you are talking about is the 2 front brake hoses and the one rear brake hose? You are not referring to the steel brake lines that travel the length of the vehicle....correct?
|That's right, they sell as a kit of 3. The cost here was about $120.00 AUD so should be around $100 US I guess. You won't be disappointed.|
|The problems described by B.H.Davis are exactly the same as I have.All the brakes showed up o.k.on the m.o.t.test rollers except the rear offside,which was sightly down the readings.The examiner said a slight adjustment on the drum break cylinder would correct it,but it is still the same.So I am hoping one of the suggestions will cure it.Any other suggestions will be appreciated.|
|Was this footbrake, handbrake or both?|
You need to remove the drum and check for any fluid contamination, which will affect both. Also shoe orientation, on both sides the 'empty' section of the shoe should be uppermost on the front shoe and lowermost on the back. Also check for uneven wear one side to another.
If only footbrake check that you can push the slave pistons back and fore a little way through the cylinder to check both are free.
If only handbrake then check the cable linkages are free, and that when the handbrake is applied the bit of the lever that the cable is attached to is parallel with the backplate both sides. If the angles are different side to side it will unbalance the braking.
|Paul Hunt 2|
This thread was discussed between 05/08/2007 and 22/08/2007
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