Quick brake question

Post by **Grahmo** » Wed Feb 07, 2007 5:33 pm

Correct me if I'm wrong, but I read this somewhere (on this forum
doubt!) On an 'normal' car with drum brakes, the master cylinder
maintains some pressure in the system to keep the shoes against
the drums. Over time this pressure is lost and if you havn't driven
the car for a while on the first press of the peddle there is a bit
more movement than normal.

Right my question. I have adjusted the brakes and bled them (am sure
there is no air in them) and I am finding that the this effect is happening
not over several days (like I remember) but in a few minutes. Is it likely
that the 'valve?' in the master cylinder is going wrong or is the problem
going to be somewhere else. I have recently replaced some drums, all
shoes and flushed out the rather old brake fluid. Oh and replaced a leaky
piston! I was rather hoping to leave the master cylinder alone!

Thanks for any help!

Post by **Willie** » Wed Feb 07, 2007 6:04 pm

Yes, a valve in the master cylinder maintains a residual pressure( around
5-8lbs) to keep the wheel cylinders in close proximity to the drums, not
rubbing just close. If you find that the first pump on the pedal goes further down than the subsequent pumps it means that the brake shoes are taking up the slack which may or may not be due to the Master Cylinder relaxing a little. The time to worry is when you press on the pedal and it continues to move slowly towards the floor.

Post by **rayofleamington** » Wed Feb 07, 2007 6:32 pm

'Minutes' is perfectly normal. Somebody probably said 'all day' but after having 16 Minors, i've yet to experience anything more than minutes.
2 of them had no time at all - due to a faulty valve in the m/c. One m/c valve had worn away and another m/c valve had been wrongly fitted at the factory.

Post by **bmcecosse** » Wed Feb 07, 2007 9:16 pm

It depends really on how 'easy' the pistons are in the wheel cylinders (if they are a bit sticky - they are not going to come back very quickly) - and on the strength of the return springs. But - if the shoes are correctly adjusted up to be 'just slightly' rubbing the drums - the effect should be very 'Minor'. Check the adjustments - especially at the rear - where you must slacken off the cables - then adjust up the shoes at the drums - and only then tighten up the cables to give the desired movement on the handbrake.

Post by **Axolotl** » Wed Feb 07, 2007 9:34 pm

where you must slacken off the cables - then adjust up the shoes at the drums - and only then tighten up the cables to give the desired movement on the handbrake

That is quite wrong. You should never need to touch the handbrake cable adjustment, unless they are very old and stretched.

The correct procedure for adjusting the rear brakes is to put the handbrake fully off, adjust the brakes with the adjuster on the back plate so they lock the drum against turning by hand then slacken off the adjuster by one click. Put the wheels back on, then pull the handbrake on without depressing the pawl release. Count the clicks. It should lock both rear wheels against hand turning after four or five clicks of the ratchet.

If it doesn't, then, and only then, should you adjust the handbrake by taking up slack (or releasing it) using the adjusting nuts on the handbrake ends of the cables.

This is all made quite clear in the workshop manual, and also in Haynes and Autobooks.

(and it works. I've done it.)

Post by **bmcecosse** » Wed Feb 07, 2007 9:44 pm

No - it's NOT wrong. You are right in that the cables should 'never' need adjusting - BUT some owners do adjust them up - and the only way to get back to the correct setting is as I suggest above. Slacken them off - then adjust up the shoes - then re-tighten the cables until the handbrake is as you would like it. Some like it short - some like it long. After doing this- there should indeed be no further need for adjustments to the cables - they should not stretch significantly.

Post by **simmitc** » Wed Feb 07, 2007 9:47 pm

Both the above posts are correct in their own way. IF (big IF) the handbrake is correctly adjusted then there is no need to slacken the cable before adjusting the rear brakes. HOWEVER there is always a good possibility that a previous owner has taken up slack by adjusting the handbrake instead of the rear brakes, which means that it is essential to make sure that the handbrake is slack before adjusting the brakes on an unknown car.

As to the pedal needing a "pump", no id definitely should not. If you mean that it normally goes down about 1/2 inch but ocassionally moves 5/8 then I wouldn't worry. If it goes nearly to the floor on the first press and only 1/2 inch on subsequent presses then either there is still air in the system or the master cyclinder needs servicing / replacing.

As ever with brakes, make sure that they are right, not "iffy" your own life and that of others depend on your brakes working when you need them.

Post by **bmcecosse** » Wed Feb 07, 2007 9:50 pm

Indeed - it's the 'previous owner' that can be the problem. Once set - the cables don't need any regular adjusting - but they do need to be set right in the first place.

Post by **Axolotl** » Wed Feb 07, 2007 9:55 pm

I'll get my coat...

Post by **Welung666** » Wed Feb 07, 2007 10:01 pm

Axolotl wrote:I'll get my coat...

Make it a thick one, it's darn chilly!!!

Post by **Grahmo** » Thu Feb 08, 2007 1:51 pm

Thanks for all the replies.

There is a good inch difference on the peddle after the brakes 'relax' so I going to get to the bottom of the problem before driving. The peddle doesn't feel spongy at all so I am pretty certain I've got all the air out. One of the front cylinders were very stiff before I cleaned them up and I suspect this was masking the problem. Now they are free they can relax back more making the effect more noticeable (at least the car stops straight now!).

I will have a go at a rear full brake adjust including the handbrake (thanks Axolotl) but I suspect I'll replacing the m/c soon.

Thanks for all the info!