Leaking pads are the bane of the woodwind players life. The biggest problem by far is the fact that so few players actually understand the nature of the problem - though many are subject to its symptoms.

In many cases the reason is down to assumption - when you buy a new instrument you assume it works, when you've had an instrument repaired you assume it works, if a cursory examination reveals nothing untoward you assume it works.
And so many players struggle on with leaky instruments believing that the lack of tone, or the hesitancy of the notes to speak is down to their technique - or the quality of their instrument. Even the most expensive instrument will play like the cheapest if the leaks are severe enough.

On this page I will explain the theory behind the action of the pads and show you why and where leaks occur, and how you can test for them. As for how you can remedy those leaks, it will be dealt with at a later date - it's quite a lengthy topic!

To some degree I've simplified the topic in order to better impart a basic understanding of the principles of the operation of the pad - there are many other factors that could be involved ( not the least of which is wear in the action ) which I might cover at a later date.

You'll need a 'test feeler' for this. There are two common options; Lightweight cigarette papers ( such as Rizla Blue ) or the cellophane wrapping from a cigarette packet. Many foodstuffs come in boxes sealed with thin cellophane - so if you don't fancy buying cigarette papers or 20 Marlboro, get yourself a box of chocolates!
Don't be tempted to use anything thicker, it will yield inaccurate results.
Using a sharp blade, cut a strip from your paper/cellophane...it helps to taper it so that your fingers can grip the widest part and the narrow tip ( not a point, leave it about 5mm wide ) acts as your feeler 'blade'. Don't make it too long, 8 to 10 centimetres is ideal.

On to the theory then!

Here we see a diagram of a basic key cup and its corresponding tone hole - minus the pad. The pad sits in the cup, and part of its thickness normally extends beyond the rim of the cup.
There is a relationship between the thickness of the pad and the angle at which the key cup comes to rest, and this angle determines whether or not a pad covers the tone hole evenly.

Here's the same arrangement with a pad in place. As you can see - the cup is square on the tone hole, the pad is too and covers the tone hole evenly all the way round. You can think of this pad as being of the 'Optimum Thickness'.

You can see perhaps that the thickness of the pad is critical. Bear in mind that the key cups are pivoted some way back down the key arm, so as the cups rise and fall they do so at an angle to the tone hole...and not straight up or down.

Take a look at this example. Same cup, same tone hole - but this time a thicker pad has been fitted. Because it's thicker than the Optimum Thickness required the rear of the pad contacts the tone hole before the front does. In other words, the cup doesn't come down square...and nor does the pad.

This example is the exact opposite, the pad is thinner than the Optimum Thickness, so the cup is allowed to travel beyond level - resulting in the front of the pad contacting first whilst leaving a gap at the back.

All good and well, and hopefully quite clear. But ( and there's always a but ) there are times when, for a variety of reasons, it isn't possible for the cup to come down square on the tone hole ( or it's just plain too much trouble for the manufacturer or repairer to correct the angle! ). In these instance it's the pad that takes up the angle, like so...

Here you can see that although the pad is sitting in the cup at a slight angle, it's still covering the tone hole perfectly. This is typically done by backing up the pad with card.
All things considered I prefer to ensure the cups are square on - using pads set at an angle doesn't make for a very even action.

So that's HOW pads leak...as for the why, well, there are a number of possible reasons.
Setting pads is something of a skill... a knack. It requires patience and accuracy, a certain amount of feel, a goodly amount of experience...and a few specialist tools.
This adds up to a costly process, and many manufacturers get around this by using clamps and heat treatment to set the pads.
The problem is that pads are inclined to absorb moisture - and to some degree the body of the pad ( typically a woven felt disc ) has a 'memory' - and this means that as soon as they're unclamped they're inclined to swell.
This results in a leak like the one shown in the third diagram - the pad swells beyond the Optimum Thickness and causes a leak at the front of the key cup.

Sometimes this phenomenon happens 'in reverse' - the pad is too thin to start with and as it expands it pushes up at the front of the cup leaving a leak at the rear.

A few other reasons for leaky pads include damage to the bodywork and tone holes/keywork, a bad repair/manufacturing job...or just plain fair wear and tear of the pads.

So let's get down to business.

Here's your home-made feeler in action!
The principle is straightforward enough; the feeler is laid over the tone hole for a centimetre or so, the cup is brought down so that the feeler is sandwiched between the pad and the tone hole and the feeler is then slowly withdrawn...noting the amount of 'grip' the pad exerts on it.
The cup is raised, the feeler moved around the tone hole and the whole process repeated.

The manner in which the cup is brought down is worthy of mention. If you press it down hard then you'll be imparting far more force to the cup than you could ever exert in normal playing - so aim to bring the cup down with the same pressure you'd normally apply in playing...or lighter, if you want to test more critically.
Consider the front of the cup to be South and test there first, then place the feeler North, then West, then East. Each time you pull the feeler out you should notice an even grip on it.

Chances are you'll find that it only slightly grips at the front of the pad, and practically tears your feeler in half at the rear! Replace the feeler if it gets all scrunched up.
The nastiest leaks are those at the North point, the back of the cup - just where you can't see them!
If you find no difference then test the in-between points - factors such as warped tone holes and uneven cups often throw leaks at the north westerly/easterly position.

If you DO find a difference then it's a fair bet that there's a leak there - or at least the potential for one. It might take you a while to get your hand in, some pads bite down differently to others and it takes a while to get a feel of what's likely to be a problem or not.
Test compound or stack keys individually and then as a group ( i.e. press the F key down, say, and check the seat of the pad on the other key it brings down ) - though this is more likely to point up errors in the action rather than the pads.

The real nail-biter is when the pad fails to grip the feeler at all! Congratulations...you've found a big leak!!