Ok, 87 C4 and in order to get my vents to blow I have to disconnect the vacuum to the Cruise Control and leave open (not cap off).  When I  hook the vacuum back up the air blows through the defrost and heat.  Replaced the check valve with no change.  If anyone has any input to help me fix this issue I would be grateful.  I know it is a vacuum issue just not sure where to look.  Been through many posts and the FSM and did not get any guidance.

Thank you in advance!

Here is info I found that looks like it may help you:

When vacuum is insufficient or routed incorrectly, the HVAC system defaults to defrost mode — this is a fail-safe in most GM vehicles from that era. Disconnecting the cruise control line may be relieving a vacuum draw or exposing a leak that restores enough pressure for the dash vents to operate.

Step-by-Step Troubleshooting & Fix:

1. Check Vacuum Source Line (From Intake Manifold to Vacuum Reservoir)

• The intake manifold supplies vacuum to a vacuum reservoir (a black plastic ball or canister).

• This vacuum reservoir supplies constant vacuum to both HVAC and cruise control systems.

• Inspect for cracks or disconnected hoses between:

  • Intake manifold

  • Check valve

  • Vacuum reservoir

  • HVAC control unit under the dash

Confirm that the check valve is installed in the correct direction and is holding vacuum.

2. Check HVAC Vacuum Line Routing & Function

• Under the dash, there’s a vacuum actuator manifold connected to the HVAC selector switch.

• If this switch is leaking or not getting proper vacuum, the dash vents won't engage.

• Check vacuum lines behind the HVAC control panel for:

  • Loose fittings

  • Cracked rubber connectors

  • Damaged hard plastic lines

Use a handheld vacuum pump (like a Mityvac) to test each actuator under the dash. Dash vents use a specific actuator — if it's not holding vacuum, that door won't open.

3. Inspect the Cruise Control Vacuum Diaphragm

• The cruise control unit has its own vacuum servo/diaphragm.

• If it leaks internally, it can drain vacuum from the system and cause HVAC malfunctions.

• Temporarily cap off the cruise control vacuum line (not leave it open) and see if the HVAC works normally. If so, the cruise diaphragm is likely bad.

4. Test All Vacuum Actuators and Lines

You should see these HVAC vacuum actuators:

• Defrost door actuator

• Vent (dash) door actuator

• Floor door actuator
  Each one can be tested individually with a vacuum hand pump to see if they hold pressure.

5. Replace Any Faulty Vacuum Lines or Actuators

Even a small crack in a hard plastic vacuum line can ruin the system pressure. Replace lines with correct size vacuum tubing if brittle or cracked.

Recommended Tools:

• Handheld vacuum pump (like Mityvac)

• Smoke tester (optional, for finding leaks)

• Basic hand tools

• Flashlight and mirror to inspect under-dash area

Quick Test Recap:

• Engine running, HVAC in vent mode, cruise vacuum line capped off → If dash vents work: the cruise control vacuum servo is likely leaking.

• HVAC stuck in defrost when cruise vacuum connected → cruise control is pulling too much or leaking vacuum.

• Cap off vacuum lines, not leave them open. Leaving them open creates an intentional vacuum leak.

🔧 Likely Fix:

Replace the cruise control vacuum servo or keep it disconnected with the vacuum line properly capped off, not open.

 Notes:

• The vacuum reservoir is often a black plastic ball or sphere mounted near the fender.

• A small check valve (usually black & white) sits in the line just before the reservoir.

• The cruise control servo is a known source of vacuum leaks when the diaphragm fails.

• Under the dash, the HVAC control switch routes vacuum to actuators based on your vent selection. If vacuum is low or misrouted, it defaults to defrost.

How to Trace Lines:

• Start at the intake manifold vacuum port (usually a large port at the rear).

• Follow the vacuum line through the check valve and toward the vacuum reservoir.

• From there, trace the lines going:

  • Through the firewall to the HVAC control switch under the dash.

  • To the cruise control servo.

Thank you so much!   I will use this today to try and track down the issue!!!!