I have noticed that occaisionally, when I go to hit the brake pedal, it seems to travel halfway to the floor before I feel any resistance.

Upon releasing and pressing the pedal again, the resistance happens where I am more used to it, up near the start of pedal travel.

It does not do this every time. Most of the time the pedal resistance is where it should be and the braking is very good when I hit the pedal quickly.

If i hit the pedal quickly the brakes seem to work fine but when I hit them gradually, the pedal goes closer to the floor and I do not seem to be getting any front braking evident by the feel of the rear brakes locking up.

I have adjusted the rear brakes and the shoes are in good condition.
I have bled all the way around and I do not have any leaks.

Also, I tried blocking the pedal down with a length of wood and found that when I did this, none of the wheels are locked or showing any resistance at all.

The car is a RHD 1992 with front disks, type 4 (?) master cylinder with the vacuum booster.

Any ideas?

Henry

If you have adjust the rear brakes "correctly" on the flats.
Bleed the system and front pads are good.

Can you also check thier is no movement with the Callipers
Check all hoses for swelling and not crimped and no leaks on the rear cylinders.
I'm guess you have no brake fluid running down the brake pedal also

But i think it will be the Boost

_________________
Stop Licking the Dog...I Don't Care Who Started It

Master cylinder seals are on the way out- put a seal kit in it before you have NO brakes.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

Check for leaks into the cabin from the brake master cylinder.
What type of brake booster is it? Early type no bleed nipple or VH44 with bleed nipple on the top? Have you fitted the brake pipes correctly to the booster (not in backwards)?
sounds like to me there is air in the booster part of the system or there is a small vaccum leak.
Check the little rubber elbow pipe from the little diaphragm under the cylinder as this does perish and cause small headaches and check the pipe that runs from the manifold to the booster as it may have the same problem or may not tight enough on the fittings.
Another thing, it could be leaking into the vaccum booster itself through the piston seal at the back of the booster master cylinder and giving no tell tale signs of leaking outside of it as its a sealed unit. When you hit them hard the doesnt have a chance to bypass the seal. Must be a small leak.
Hope some of this is useful. Keep us informed of your progress.
Cheers

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http://www.youtube.com/watch?v=nDMkVq1jRGU

gafmo, the rears are correctly adjusted as you say, "on the flats".
The front pads are good, and the calipers are good and firm, no movement.

I did just replace the rear arm pivots and had the rear hoses clamped while doing this. All is well at the back because they seem to lock up when the fronts don't.

No brake fluid running anywhere and the level is staying constant.

Looks like it may be the Master Cylinder seals as drmini says.

It makes sense I think. If the seals are worn, they could be 'squishy'. When lightly pressing the pedal, they aren't forced to fill the bore and seal, when the pedal is pressed firmly, they will seal and that is why the brakes work then.

I found the kit I think I need is a GRK1032.

You don't always get leaks and fluid loss- if the top seal gets worn, fluid bypasses it under light pedal pressure, and the pedal goes down. The bottom seal is there to stop leaks from the bore...

_________________
DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

BALLISTIC, no leaks into the cabin and the fluid level has stayed very constant.

It is a later booster mounted to the Master Cylinder. It has not been disconnected or dismantled.

I can simulate this problem with the car running or not so I don't think it is vacuum related.

Thanks for all your help, BALLISTIC, drmini, and gafmo!

Henry