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MG MGB Technical - Brake pedal is hard...
A friend of mine bought a MGB 1971 last week and when compare to my '73 applying brake you must push hard on brake pedal and braking is a lot less efficient than on my car. I tought it was some pistons at wheels that are stuck but when you jack the car you can turn easily wheels and it brakes when depressing pedal. What could be the problem..
|Jean Guy Catford|
|My guess is that the 71 car does not have a brake servo fitted but the 73 car does.|
Without a servo it requires a lot more effort to apply the brakes. The servo uses the vacuum pressure from the inlet manifold to increase the breaking force hence less effort is required to produce the same amount of braking.
|Yes, the servo does wonders, my wife can only drive my 80B, not the 71B, thats for me...|
| Hi Jean-Guy,|
A hard brake pedal for MK1 is provibg sanity of the system. If less efficient, tell your friend to swap its discs; a plenty of difference on mine just after changing them: I can even lock the front wheels when depressing the system strongly and I have really strong breakes
I didn't check disks but it could be the problem.
No guys, servo brakes were unavaliable in these years for a MGB.
|Jean Guy Catford|
|Servos were often fitted after sale, just have a look under the bonnets. Mine doesn't have a servo and only today I locked it's skinny tyres at 40 mph without realising I was that close to the limit of grip. If fitting a servo do be careful as the rest of the car may quickly become the limiting factor in actually stopping. That is, if you still have the original tyres etc you really need all the feel you can get.|
Lucky you were in GB as in america with left-hand steering a servo was a pain in he neck to install. Thats why it was not an common option here.
P.S. But it doesn't solve the hard-rock brake pedal in the '71...
|Jean Guy Catford|
Check the rear wheel brake adjusters. If they aren't tightened up, braking will only be done by the front discs making it a lot less efficient.
|Loose rear brake adjusters will cuase the pedal to feel soft for the first inch or so of travel. A hard pedal can be from a combination of things such as old hard rubber seals on the calipers and wheel cylinders. Old swollen brake hoses can also cause increased pedal effort. Lastly, and all three of these issues are probably causing your problem, swollen and softened rubber seals in the master cylinder will also cause increased braking effort. I would also speculate that your clutch is less difficult to depress than the one in the '71 as well, although that would be less noticeable as there are far fewer components with issues to "compound" the problem. Speaking of "compound" how old are those brake pads and shoes....|
|The brake pedal should not be rock hard, that suggests a brake problem at one or more of the wheels. I'd recpmmemd a complete overhaul of the front brake calipers, fit new rubber seals and dust seals and same for the rear. Just a good mainteance practive in any case. Also check type of brake pads and condition of rotors.|
I have both a 67 single brake master cylinder and 74 dual master cylinder and both cars are easy to brake. No excess pressure required.
|The rear brake lines haven't been mentioned. If the car has been towed the rear brake lines on the axle assembly may have been mashed flat by straps used to tie the car for towing. One MGB I purchased had the brake lines flattened, it was also hard to brake the car. A differential I purchased had flattened rear brake lines. That's two out of four for me. Am I that unlucky? Check the rear brake lines.|
Disc brake pads have a varity of compounds and can make a difference in brake feel. Old pads and rotor can glaze and make a car hard to stop.
I will have to verify the whole braking system to find the trouble origin...
Jeff: brake pads are 4 years old and from standard asbestos type and in good shape as the car was used only during summer.
|Jean Guy Catford|
| Of course, at front disc replacement step, I too renewed the two calipers with new rubbers and pistons.|
In the case of adding a servo, I have been told to reduce rear brakes cylinders by mini ones to avoid rear wheels blocking. Is that true ?
|There are conversion rear brake cylinders available in the GT sizing that reduce rear brake bias. I redrilled the backing plates and used the GT cylinders. I am very happy with the way they function. The rears no longer lock up early during emergency stops.|
|Why would adding a servo change the brake bias?|
A stock 'B is adequately front biased - unless you increased weight transfer with much stickier tires or a bigger engine your stopping distances will increase if you reduce the rear bias.
|I don't think that the servo has anything to do with bias, but if you read through the archives, many people have changed their brake bias for the better with the GT cylinders. It DOES improve braking, and the stock bias is NOT that good. There are very few Bs out there with stock tires, hence the need - and if its apparent with larger tires, its also there with smaller ones. The problem would lie in all 4 locking up "prematurely."|
|Jeff, I confirm your point of vue. Epoch technical articles do show up clearly this brake improvements that Gt rear cylinders wheels carry on. MGOC sells "MGB rear wheels cylinders" that help delaying the rear wheels locking and further more they help in improving front/rear brake balance.|
|I got a 80B with servo, a 71B and a 67 GT without, and i dont see much of a diference with servo, maybe a little more pressure on the pedal..not noticeble.|
Any link to epoch article?
If asbestos! pads I would change them.
The MGOC "uprated" may be smaller cylinder (not checked).
The servo will not effect brake bias only leg effort.
Brake balance is effected by
Disc effective diameter
Caliper piston diameter
Pad/Shoe friction cof
Factors that will increase front bias
Increased front disc diameter
Increased front brake pad cof
Increased front caliper piston diameter(s)
Decreased rear caliper piston diameter
Lower center of gravity
More weight on rear axle
Less weight on front axle
Less sticky tires
lower traction conditions
|Be careful with this stuff guys, there's a lot of misinformation out there, esp on the web. Most of you know the consequences (which are deadly), but for those that don't: |
Increase the rear bias too far and your rear tires will lock up before the front. In a panic stop you'll spin.
Decrease the rear bias too far and your stopping distances will increase as the rear tires don't do their share. This is often the consequence of "uprated" front brake packages - it feels better 'cause there's more leverage on the fronts, but you get worse performance.
It's the tires that stop the car, not brakes. Upgraded brake pads & discs might reduce fade and perhaps improve feel, but they won't stop you quicker and can hurt the balance.
Race drivers have bias preferences from wildly front to wildly rear because they often trail brake. On the street a safer front bias is prefered.
From Paul's list - smaller rear cylinders will reduce rear brake bias - less tendency for the rears to lock but longer stopping distances. Larger rear cylinders will increase the tendency for the rears to lock. Stickier tires can change weight transfer, as will lowering the car, so you might need to change brake bias in that case. Hard to do intuitively, consider having a read of Fred Puhn's "Brake Handbook".
If your rear tires lock before the front in a stock 'b with stock-ish grip tires there's something else wrong or changed. And don't believe anecdotal "improved brakes" stories, look for documented stopping distance improvements and make sure they apply to your car (as in similar tires, etc).
I've always found the 'B braking system to be superior to the over boosted over front biased dead feeling modern cars I'm sometimes forced to drive (admittedly low end cars, but then so is the B). My 'B stock brakes are easy to lock and to modulate.
|Paul, I caught that info in the excellent book " The MGA B & C" by Chris Harvey, chapter "Modifications " page 184. If you have an Email adress; I can post it you.|
Hopefully email will show.
Lots of variation and as Mike has pointed out "illusion".
| Hi Mike,|
I can insure you that Mr. Harvey is really well known in the British classics world. His statements do insist
in reducing rear break strneght to avoid rear wheels lock by converting the stock rear cylinder on a B by mini's ones. Provided he wrote that in his book, I bet it's absolutely reliable. Regards
|A few numbers ball park|
OE brakes 60% Front 40% Rear
Upgrade front or degrade rear now 70% Front 30% rear
Maximum front tyre grip at point just before lock up 1500lbs
So OE Front 1500 rear 1000 total 2500lb of stopping force
Upgrade Front 1500 Rear 650 total 2150
Nearly 15% reduction in stopping force!
I also run some quick calcs on the AP racing set up for mgtf which should generate more grip than MGF
The AP Racing 304mm fronts use 32mm (1.25ins) pistons as opposed to std 48mm, reducing clamping, but there is more torque from bigger disc.
Appox calcs show std MGF Front rear split 62/38 compared to AP racing on MGTF 65/35.
|I'm sure he's right Renou, but be careful to compare apples to apples - is he talking about a stock or modified car? If modified, in what way?|
While it's possible, it seems unlikely that the engineers would've allowed the 'B to leave the factory with too much of a rearward brake bias. It's considered too dangerous for untrained drivers. Certainly my roadster 'B doesn't lock the rears first with stock brakes and uprated tires - I had a chance to try them out quite a bit at a performance driving school recently.
|Yes Mike, I absolutely agree with you. I meant stock brakes were improvable by fitting a servo provided you reduced the rear brake power by fitting Mini cylinders instead of stock ones.|
|I have '67 gt with out the servo. Braking required lots of pressure. The early gt's have a smaller rear wheel cylinders than the later gt's. |
I replaced the cylinders with the larger style and it helped lower pedal pressure a little bit.
I replaced the front brake pads Hawk Ferro Carbon pads, they put out very little dust and have a higher friction cf than the green pads everyone uses.
Total result: substantially less pedal pressure, but still more pressure needed than with the servo brake system.
Given the fact that the new pads are higher cf than stock and that the rear brake shoes are still lined with ordinary linings (lower cf), the rear brakes even with the larger cylinders (larger creates more brake pressure than smaller)still don't lock up under hard braking. Probably a roadster could use the GT rear brake cylinders without any problem.
|Aren't you lucky that your brake pedal is rock hard. At least it confirms that the system is bled properly. Just check that you can easily push the car on a flat surface and also that if you jack the car up and rotate the wheels individually they stop immediately the brake is depressed and free immediately on release.|
The brake servo does NOT improve the braking. It only reduces the pedal load and bearing in mind that it only has a 1.6/1 ratio is not really worth bothering about. Don't try to alter the brake bias front to rear as this will only lead to further problems. On the assumption that the tyres are all the same size the original brake proportions will remain correct.
Changing discs will make no difference whatsoever to the feel of the pedal except to say that due to the fact that new pads are a must when discs are fitted the system will initially feel softer. It will harden up over about 500 miles but this wont be noticeable. The only other thing is the grade of pad. I'd stick to the standard pad and shoe material for a road car. Any attempt to alter one or the other will again result in an unbalanced system. If the pads and or shoes are not standard and of a hard grade this will affect the feel of the pedal and also result in considerable extra force being needed to stop the vehicle.
This thread was discussed between 25/07/2005 and 01/08/2005
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