Could be a number of things, you didn't list any specific symptoms but the most common reasons would be related to either the ignition or fuel system. I would recommend starting out by checking the Spark plugs, plug wires, distributor cap and rotor button(not applicable if it's equipped with a distributorless ignition), ignition coil, fuel filter, and running either HEET and some type of injector cleaner through the gas tank. I'd try the HEET and Injector cleaner first, because it's cheapest. Then I'd look at everything else.
Sounds like a bad master cylinder, if you push down on pedal with light preasure and pedal goes too far down then push hard and quick to see if it will hold if so cylinder is leaking
Pushing the brake pedal down to hard.
The gas pedal could be difficult to push down for a few reasons. One reason that it could be hard is due to a pinch in the cable. It could also be an issue with the gas linkage.
power booster behind master cylinder sounds like they may be the problem
if the brake pedal is hard you might first want to check if the mineral oil is full, get the castrol mineral oil for jaguars with the attached pump. Make sure the oil level is green with a black dot, if is clear with a black dot then the container is empty and it needs oil.,, Good luck bud..!!
You might not be holding the brake pedal down hard enough or there may be a problem with that gear.
A hard brake pedal can be caused by several different factors. It can be as simple as the brake pedal getting caught on something to the power brake booster being bad. You could also have a brake pad or shoe that is sticking, causing the brake fluid to boil. When brake fluid boils, it expands and can cause the hard brake pedal that you feel. A brake pedal that is hard to press can also be caused by a build-up of water in the brake line. The water in the brake line lowers the boiling point of the brake fluid. A hard brake pedal can also be the result of a restriction in the brake line, such as a bent brake line or foreign material in the brake fluid. The most common cause of a hard brake pedal is the sticking brake pad or shoe, causing the brake fluid to boil. Tracking down what is causing the hard brake pedal can be frustrating, to say the least. Take the vehicle for a drive and when you come back, feel your brake lines near the wheels. Caution: The line may be very hot! If one is extremely hot, you may have just found the culprit causing the hard brake pedal.
Start the engine and then push the brake pedal down. If it is soft and easy to push then the booster is okay. If booster is BAD the brake pedal would be hard to push.
With a foot pedal. The pedal is usually chain driven and utilizes a beater (soft or hard) which rotates when the chain is pulled down by the foot to strike the bass drum.
Adjust or replace the stoplight switch at the upper end of the brake pedal under dash area
Start the engine. Place you foot on the brake pedal, and press fairly hard and hold the pedal down. If the pedal very slowly, moves toward the floor any at all, the master cylinder is defective. Replace it immediately!!!!!
Verify that the vacuum source for the brake booster is working. Might need a new booster.