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MG Midget and Sprite Technical - Ditching a old servo

I have a 1971 sprite and I guess due to lack of use I am forever having brake issues (sticking calipers, leaking cylinders etc.)

I have bitten the bullet and replaced both calipers with reconditioned units, and while I was at it I decided it was time to remove a hideously installed servo that a previous owner had added. I wasn't apparently leaking but was looking knackered so I removed it and fabricated a new brake pipe and routed and secured it properly from the master cylinder to the union block.

I've bled the brakes, and have now got the brakes working on the pedal, after about 1.5-2" of travel.

Having driven it on a sevo for 10 years, I don't know what standard brakes feel like or what is normal. How much travel should I expect in the pedal? with regard to the pedal coming back, it seems stiff: is there a grease nipple somewhere? is there a spring that is busted on mine? does it return purely on the push back off the pads when moving?

Any advice gratefully recieved!

thanks,
John
John Neal

There should be a return spring on the pedal. This should be visible looking under the dash. There is also one on the clutch pedal.

There can be a problem with the pedal seizing on the pivot bolt. Try turning the bolt and also a few drops of oil between the two pedals...after removing the cover plate.
Dave O'Neill 2

certainly with less use the pivot bolt would be my suspect.
if you can take the whole lot a part and clean the bolt and holes with fine sandpaper (320 grit) and grease exesivly before reasembly
Onno Könemann

Thanks guys. Had the cover plate off, springs are still intact and clutch pedal returns fine - looks like the brake pivot bit is rusted solid. Have liberally smothered it with penetrating oil. Expect I shall have to have the pedal box out next weekend :-(
John Neal

turn the bolt with a wrench back and forth and hold the pedal that should give a quick fix
Onno Könemann

Thanks. Had a look last night and it is all free now and moving. Still doesn't return back fully though. I've replaced the return spring with a new one, and greased it all up.

I think I'll bleed it all down again to make sure I have got rid of all the air then run it a bit and see if it loosens.

I'm guessing an optimum non servo system has say 1 inch of travel on the pedal before hitting brake resistance, will lock up if you stand on the brakes, pulls up straight then releases again so the friction material is just off the discs when you are moving again?
John Neal

"just off the discs" needs qualifying.

There is nothing that actually pulls the pads clear of the discs - other than the return spring in the M/C possibly creating some suction. The pads are simply scuffed away by the revolving discs. This is one of the main advantages; it requires virtually no travel and therefore not much brake fluid movement to get the disc brakes working. Unless, if there is too much run out on the disc (warped discs or loose wheel bearings) the pads are pushed further away and more pedal movement is needed to apply them.

The real key to good braking is properly adjusted rear shoes - not that the rear brakes do much braking but they account for a lot of the pedal travel when badly adjusted!

Guy Weller

Ah yes that's what I meant. Discs and pads seem good.

So how much travel should I be looking at in the pedal (from fully up on pedal box, return spring closed)?

Good point about the rear hubs - will double check they are set perfect before I carry on.

Thanks,
John
John Neal

John -
There is a necessary few thou clearance at the MC pushrod, and a similar amount inside the cylinder for the seals to clear the ports. This gets multiplied up to maybe 1/4" max at the pedal if the clevis pins are not worn.

"Still doesn't return back fully though" is worrying, because if the seals don't clear the ports inside, the system will build up pressure as it warms, will drag and eat your brakes. Also if the seals are not clear of the ports the system can't compensate correctly for increased travel required by pad wear and you will get low pedal. This can also lead to bleeding nightmares; there will be a constant low level vacuum and an air bubble in the circuit after you've closed the bleeder and the pedal returns, since the system can only refill by sucking fluid past the MC main seal. It should relieve this and final fill through the ports once the MC piston returns to rest position.

When things like this get seized on pivot shafts, it is easy to get them free but they usually retain stickiness at the ends of travel. You should take the pivot bolt out and clean & grease everything up. The last degree of return must be as free as the rest of the travel.

The pads are drawn back by the twisting of the square section caliper piston seals, just a couple thou. Guy's point re sloppy wheel bearings is right on, this can add a lot of travel, which tends to go away when you stomp on the brakes whilst stopped, but comes back after going around a few corners.

If you lock the rear brakes hard with the adjusters, and all air is out, there will be essentially zero pedal movement past the preceding. Back the adjusters off to normal and any added pedal travel is down to rear brake movement; it should be slight once the rear shoes are bedded in.

I figure if total travel is more than 1" something is off, and 2" requires fairly immediate attention.

FRM
Fletcher R Millmore

Thanks for that Fletcher - I will go through this on the car this weekend and see if I can work it out.

I've got recon calipers, recon master cylinder and new rear cylinders so fingers crossed I just need to adjust it properly
John Neal


Results from the Essex enquiry;


Offside rear shoe poorly adjusted, Nearside slightly out (fixed)

Big air bubble on nearside front (fixed).

Took it out for a shakedown, brakes now much improved - pulling up straight and sharp. Need to bleed some more all around to get it perfect - still feels slightly spongy.

Now all I need is a blanking nut to block out the manifold where the Servo intake pipe went. Anyone knows who will sell one? Normally use AH spares, will give them a ring




John Neal

John -
Great! If you have new rear shoes, it will be slightly spongy until they bed in, which can take a while since they do so little work. You can speed it up by using the handbrake to slow the car when convenient and safe. Then readjust.

FRM
Fletcher R Millmore

I seem to recall that the servo take-off on the manifold is the same thread as the bypass pipe on the cylinder head. There is/was a blanking plug fitted to A+ heads. I have a pack of them somewhere.

modgetbitz AT gmail DOT com
Dave O'Neill 2

This thread was discussed between 27/02/2010 and 08/03/2010

MG Midget and Sprite Technical index

This thread is from the archive. The Live MG Midget and Sprite Technical BBS is active now.