Testing Booster
Most power brake systems in autos are vacuum-based, using the difference between engine intake vacuum and atmospheric pressure as their power source. To test your brake booster, pump the brakes several times with the engine off to deplete stored vacuum. Turn on the engine while pressing lightly on the brake pedal. You should feel the pedal fall away a bit and then become firm, but not hard. If you feel nothing at the pedal when the engine starts, your brake booster is not functioning. On a side note if you went to a new cam that produces low vacuum it can cause braking issues.
Another Test
Booster problems may indicate failure of the booster unit itself or problems in the vacuum system that powers the booster. A vacuum system problem may leave you with only partial boost. To test for vacuum problems, start the engine and run it to medium speed, then turn off ignition and take your foot off the gas. Wait about a minute and a half and then apply your brakes. You should feel the vacuum boost on at least two brake applications. If you do not, you may have a vacuum system leak or bad vacuum check valve.
Sounds like it's more of a Master Cylinder issue. This is a easy replacement, you just need to bench bleed the unit to make sure you get all the air out.
Symptoms
Master cylinder failure symptoms include a brake pedal that slowly depresses when pressure is applied. Other signs include a lack of any braking action until the pedal is pressed against the floor.
Over time, the piston used to power the hydraulics in the braking system's master cylinder circuits can wear down, or the piston seal between the cylinder wall and piston can fail. Piston wear or seal failure can lead to brake cylinder failure. A brake line or hose leak can lead to wet spots beneath the car, along with a low fluid reservoir in the master cylinder and the activation of the brake warning light. A kinked brake line or debris in the brake fluid can prevent hydraulic pressure from releasing back to the master cylinder, which can cause brake pressure to drag.
If you decide to replace both units it's not really that hard. You will need to get under the Dash to remove the 4 bolts and to disconnect the brake pedal linkage. Remove the 2 brake lines from the master cylinder and it all comes out. About a days work to get this done if it's your first time doing this.
