Bedding in rear brakes with wheels off the ground

[DavidSWP]

Hi, I'm trying to do this with my GT6 because its got no MOT and I'm in catch 22 - can't bed in the breaks without driving it and can't drive without MOT.

So I've lifted the back off the ground and just tried using the brakes while the wheels were spinning in 1st gear with no throttle (idling with choke about 1400 rpm.

I didn'r really know what to expect but the brakes did not completely stop the drums from spinning or make the engine stall.  It only reduced the rpm down to about 1000 rpm under full braking.  I'm not entirely surprised because in 1st gear it obviously has a lot of torque and the weight of the car is not on the wheels.  I'm just wondering if anyone else has tried this and if I would possibly be better trying it in higher gear?!?  Could I do any damage doing this?

Cheers,

Dave

[Rich_s]

I had the same problem when put my spitfire on the road, we explained to our local mot guy but in the end it went through the brake test fine, i did the same as you with the back end in the air and the brakes easily stalled the engine, granted mines only a 1500 compared to the 2000 in the GT6 but your brakes should be able to stop the engine.

Are the rear brakes adjusted up enough on the rear, normally i do up the adjuster until the wheel wont turn then back off 1/4 turn at a time until they turn as required. If the drums are clean and the shoes new i would have thought they should stop the engine even without being bedded in.

I presume the cylinders are activating and the shoes moving when the brake is applied?

rich

[CharlieB]

DavidSWP wrote:

....can't drive without MOT.

Of course you can drive it to the MOT provided you've booked it, take the long way round perhaps?

[Doug Paterson]

Even the least 'run in' shoes should make a trivial job of stalling a ticking over engine (choke or not).

I think there must be adjustment or other problems with the rear brakes.  

In the past helping out a mechanically challenged pal with an MG midget I discovered that his rear brake cylinders had all but siezed.  On one wheel it was clear that the pads had not touched the drum for a LONG time.

I'd suggest whipping off your drums and checking that pressing the pedal is actually having an effect on the shoes.

EDIT - go through the adjustment steps detailed by other posters before the above......

[Steve Cureton]

If the brakes are adjusted properly you don't need to bed them in. Now you have operated the brakes the shoes should be nicely centered so just do as already suggested, tighten the adjuster right up and then back it off until the drum rotates with very little binding. Make sure the handbrake is disconnec ted while you do it as that can skew the results - but don't forget to re-connect it and adjust it afterwards!

[DavidSWP]

932 wrote:

I had the same problem when put my spitfire on the road, we explained to our local mot guy but in the end it went through the brake test fine, i did the same as you with the back end in the air and the brakes easily stalled the engine, granted mines only a 1500 compared to the 2000 in the GT6 but your brakes should be able to stop the engine.

Are the rear brakes adjusted up enough on the rear, normally i do up the adjuster until the wheel wont turn then back off 1/4 turn at a time until they turn as required. If the drums are clean and the shoes new i would have thought they should stop the engine even without being bedded in.

I presume the cylinders are activating and the shoes moving when the brake is applied?

rich

Hmm interesting.  Did your brakes stall the engine from the start or only after you'd bedded them in?  Did you have the wheels bolted on?  The rear brakes on mine have new shoes and cylinders and freshly bled.

Mine has self adjusters and its hard to tell if they are working but the brakes are definitely engaging both using the handbrake and using the pedal (hydraulic cylinders) because the rpm fell quite a lot and the drums were red hot when I touched them afterwards.

[Doug Paterson]

David

Can you expand on what you mean by 'self adjusters'?

Never seen this on a Spitfire/GT6 and I'm wondering if something is being missed here.

In any event .... even the most rubbish shoes if engaging firmly with the drum should lock it easily with wheels up in the air.

[TimW]

Doug_P wrote:

David

Can you expand on what you mean by 'self adjusters'?

Never seen this on a Spitfire/GT6 and I'm wondering if something is being missed here.

In any event .... even the most rubbish shoes if engaging firmly with the drum should lock it easily with wheels up in the air.

self adjusting rear brakes were fitted to the late MK3 GT6 swing spring and are SH*T

[Doug Paterson]

339 wrote:

self adjusting rear brakes were fitted to the late MK3 GT6 swing spring and are SH*T

Ah ..... perhaps a clue  :)

[DavidSWP]

Doug_P wrote:

David

Can you expand on what you mean by 'self adjusters'?

Never seen this on a Spitfire/GT6 and I'm wondering if something is being missed here.

In any event .... even the most rubbish shoes if engaging firmly with the drum should lock it easily with wheels up in the air.

They are on the late gt6 and instead of having the screw to turn to adjust the distance between the shoe and the drum, the top of the cylinder has a ratchet machanism.  Its hard to explain, but every time you put the handbrake on, a lever moves inside the brake drum and engages with the toothed ratchet unscrewing the top of the brake cylinder and making it longer therefore pushing the shoes closer to the drum.

I had the wheels off (just the drums spinning).  I don't know it that would make much difference though.  

[DavidSWP]

339 wrote:

self adjusting rear brakes were fitted to the late MK3 GT6 swing spring and are SH*T

Yes they are, I'll take a look but they are definitely working to some extent because when I first sealed up the drums the rear brakes didn't work at all unless I cranked the handbrake loads of times, then they started to work.

[Doug Paterson]

DavidSWP wrote:

I don't know it that would make much difference though.  

This would make zero difference.

I think the previous comment that self adjusters were poor may be a clue.  It is certainly sounding there is a problem (adjusters, siezed piston, air in fluid or similar).

Investigation time .... good luck.

[DavidSWP]

Doug_P wrote:

This would make zero difference.

I think the previous comment that self adjusters were poor may be a clue.  It is certainly sounding there is a problem (adjusters, siezed piston, air in fluid or similar).

Investigation time .... good luck.

I'll have another go with the adjusters, the cylinders are brand new though and extremely unlikely to be seized (and the do work to some extent).  I bled it today and pretty sure there's no air in it - not that that would affect the handbrake anyhow.

[Doug Paterson]

DavidSWP wrote:

I'll have another go with the adjusters, the cylinders are brand new though and extremely unlikely to be seized (and the do work to some extent).  I bled it today and pretty sure there's no air in it - not that that would affect the handbrake anyhow.

You're absolutely correct - bleeding would not affect handbrake.  Defo something wrong tho.  

How firm is the pedal feeling when you press it?  Hard/soft/spongy?  Should be very firm.  

[esxefi]

david,
if you do decide to bed the brakes as you suggest remember to keep an eye on the coolant temp as the engine will be under load similar to normal driving but with no airflow through the rad.

[DavidSWP]

Doug_P wrote:

You're absolutely correct - bleeding would not affect handbrake.  Defo something wrong tho.  

How firm is the pedal feeling when you press it?  Hard/soft/spongy?  Should be very firm.  

Its pretty firm.

[Rich_s]

How much of the shoe is making contact with the drum, if you lightly sand the shoe so all the same colour then run the engine and apply the brakes to try and stop it, then look at the shoes, might give an indication of a problem if they are not all making contact, obviously they wont make perfect contact as not bedded in but something that may point you in right direction.

Referring back to an earlier post, i was getting the engine to stall with new shoes and drums without them being bedded in, the wheels were on as well. Once the car was on the road the brakes were another matter, but in the air they worked :)

rich

[DavidSWP]

932 wrote:

How much of the shoe is making contact with the drum, if you lightly sand the shoe so all the same colour then run the engine and apply the brakes to try and stop it, then look at the shoes, might give an indication of a problem if they are not all making contact, obviously they wont make perfect contact as not bedded in but something that may point you in right direction.

Referring back to an earlier post, i was getting the engine to stall with new shoes and drums without them being bedded in, the wheels were on as well. Once the car was on the road the brakes were another matter, but in the air they worked :)

rich

I didn't think that the whole shoe made contact anyway, isn't it only the end of the shoes that get pushed out on the wheel cylinder end?

Edit: I've put a picture of a brake drum, cylinder and shoes below - its not from a GT6 but the layout looks the exact same.  But what I mean is, its only the top part of the shoes which are pushed out into the drum by the cylinder (top of the photo).  The very bottom of the shoes don't move because they are just pressed against the fixed separator.  Or have I got that completely wrong?

[esxefi]

yes,look at the linings in relation to the steel shoe.the torque reaction acts like a lever and drives the shoe into the drum in a wedging action.if fitted the wrong way round the rear brake efficiency will be drasticly reduced. :)

nb although the handbrake mech should mean they cannot be fitted incorrectly.

[herald948]

DavidSWP wrote:

But what I mean is, its only the top part of the shoes which are pushed out into the drum by the cylinder (top of the photo).  The very bottom of the shoes don't move because they are just pressed against the fixed separator.  Or have I got that completely wrong?

Not completely wrong, no. But don't forget that the wheel cylinder will (should be able to) slide fore and aft, and the ends of the shoes will slide up and down where the contact the wheel cylinder and whatever that bottom pivot point is called on the self-adjusting brake setup. So the shoes will quickly "conform" to the arc of the drum.

[Steve Cureton]

I don't know about your car, but many cars with self-adjusting brakes have a small hole in the backplate though which you can insert a screwdriver and operate the mechanism manually.

[Slowdown]

If you ease the drums away from the backplate you can just get a screwdriver in the gap to click the adjuster round.  Move it a few clicks, push the drum back, hit the brake pedal a couple of times to centralise the shoes, and turn the drum to see if the brakes are rubbing.  If not, ease the drum off slightly again and repeat the process.  Once you get the brakes just rubbing, that's as good as you'll get, but be carful not to go too far so that you lock the drum.  If you do that, you'll need to be cute to get the drum off!

[DavidSWP]

It seems a lot better today.  I'm not sure what did it, I noticed one of the drum covers was slightly loose so tightened the screws up - maybe the shoes were making it wobble about on contact and not mating with the surface properly.  I also put the wheels back on because I thought maybe the extra weight would help.  Anyway now it easily stalls the engine at normal idle of 800-1000 rpm and will also stall it at 1400-1500 rpm but you have to press with a lot of force.

One other thing is, my drum covers are only held on with one screw each rather than the normal 2 - one of then has a broken off screw shaft in it which would need drilling out and possibly retapping.  Is this worth doing do you think?

[willcolumbine]

Those screws only hold the drum on to stop it falling off when the wheel is off. They're not in any way load bearing! It should be fine with just 1 - indeed I have just fitted new disks to my VW golf and they are only held in place by 1 screw.

[herald948]

Worst case scenario: I've never had a problem drilling out a broken-off screw and have yet to have to retap the hole other than to clean up rusty threads. So go for it if you feel so inclined!