<PAGE>   1
                                                                     Exhibit 4.5


                               THIRD AMENDMENT TO
                           REVOLVING CREDIT AGREEMENT

      This Third Amendment to Revolving Credit Agreement, dated as of December
29, 1999 (the "Amendment"), by and between (a) TRANSPRO, INC., a Delaware
corporation (the "Parent"), ALLEN HEAT TRANSFER PRODUCTS, INC., a Delaware
corporation ("AHTP"), AHTP II, INC., a Delaware corporation ("AHTP II"), EVAP,
INC. (f/k/a EI Acquisition Corp.), a Texas corporation ("EVAP" and collectively
with Parent, AHTP and AHTP II, the "Original Borrowers"), GO/DAN INDUSTRIES,
INC., a Delaware corporation ("GDI") and A/C PLUS, INC., a Texas corporation
("AC" and collectively with GDI and the Original Borrowers, the "Borrowers"),
(b) BANKBOSTON, N.A., a national banking association and the other lending
institutions listed on Schedule 1 of the Credit Agreement (collectively, the
"Banks") and (c) BANKBOSTON, N.A., as agent (the "Agent") for the Banks,
amending certain provisions of the Revolving Credit Agreement dated as of July
30, 1998 (as amended and in effect from time to time, the "Credit Agreement"),
by and between the Original Borrowers, the Agent and the Banks. Capitalized
terms used herein and which are not otherwise defined shall have the respective
meanings ascribed thereto in the Credit Agreement.

      WHEREAS, the Borrowers have requested that the Banks agree to amend the
terms of the Loan Documents in several respects as hereinafter more fully set
forth; and

      WHEREAS, the Banks are willing to amend the terms of the Loan Documents in
such respects, upon the terms and subject to the conditions contained herein;

      NOW, THEREFORE, in consideration of the mutual agreements contained herein
and for other good and valuable consideration, the receipt and sufficiency of
which are hereby acknowledged, the parties hereto hereby agree as follows:

      Section 1. AMENDMENT TO SECTION 1.1 OF THE CREDIT AGREEMENT. Section 1.1
of the Credit Agreement is hereby amended as follows:

            (a) The definition of "Borrowing Base" set forth in Section 1.1 of
            the Credit Agreement is hereby amended by (i) substituting the
            percentage "85%" for the percentage "80%" contained in clause (a)
            therein, and (ii) substituting the percentage "75%" for the
            percentage "80% contained in clause (c) therein.

            (b) The definition of "Eligible Accounts Receivable" set forth in
            Section  1.1 of the Credit Agreement is hereby amended by adding the
            following to the end thereof:

                  "; and (x) that are not due from any single account debtor if
                  more than thirty percent (30%) of the aggregate amount of all
                  Accounts Receivable
<PAGE>   2
                  owing from such account debtor would otherwise not be Eligible
                  Accounts Receivable."



      Section 2. AMENDMENT TO SECTION 2.3.2(a) OF THE CREDIT AGREEMENT. Section
2.3.2(a) of the Credit Agreement is hereby amended and restated in its entirety
to read as follows:

            (a) Unless terminated earlier pursuant to the provisions of
            this Section 2.3, on each of the dates set forth in the table below
            (each such date being referred to as a "Reduction Date"), the Total
            Commitment shall be automatically reduced (or in the case of January
            21, 2000, increased) to the amount set forth opposite such date in
            the column headed "Commitment Amount" set forth below, as such
            Commitment Amount may be adjusted and in effect from time to time
            pursuant to this Section 2.3 whereupon the Commitments of the Banks
            shall be reduced (or increased) pro rata in accordance with their
            respective Commitment Percentages:

<TABLE>
<CAPTION>
                        REDUCTION DATE            COMMITMENT AMOUNT
                        --------------            -----------------
<S>                     <C>                       <C>
                        January 21, 2000             $75,000,000
                        November 30, 2000            $70,000,000
                        December 31, 2000            $65,000,000
                        March 31, 2001               $63,500,000
                        June 30, 2001                $62,000,000
                        September 30, 2001           $60,500,000
                        December 31, 2001            $59,000,000
                        March 31, 2002               $57,500,000
                        June 30, 2002                $56,000,000
                        September 30, 2002           $54,500,000
                        December 31, 2002            $53,000,000
                        March 31, 2003               $51,500,000
                        June 30, 2003                $50,000,000
</TABLE>

            On each Reduction Date there shall become absolutely and
            unconditionally due and payable, and the Borrowers hereby absolutely
            and unconditionally, jointly and severally, promise to pay to the
            Agent for the account of the Banks, the amount by which the sum of
            the aggregate principal amount of all Loans outstanding plus the
            Maximum Drawing Amount of all Letters of Credit and all Unpaid
            Reimbursement Obligations exceeds the Total Commitment after giving
            effect to the reduction of the Total Commitment as set forth herein.
            No reduction of the Total Commitment may be reinstated.
<PAGE>   3
      Section 3. AMENDMENT TO SECTION 10.1 OF THE CREDIT AGREEMENT. The table in
Section 10.1 of the Credit Agreement is hereby amended and restated in its
entirety as set forth below:

<TABLE>
<CAPTION>
                     PERIOD                             RATIO
                     ------                             -----
<S>                                                   <C>
         April 1, 1999 - March 31, 2000               3.50:1.00
         April 1, 2000 - June 30, 2000                3.25:1.00
         July 1, 2000 - March 31, 2001                3.00:1.00
         April 1, 2001 and thereafter                 2.75:1.00
</TABLE>

      Section 4. ADDITION TO SECTION 10 OF THE CREDIT AGREEMENT. The following
new Section 10.6 is hereby added to the Credit Agreement:

      "10.6 Minimum Availability.  The Borrowers will not at any time prior
      to January 1, 2001 permit the sum of the outstanding amount of the
      Loans plus the Maximum Drawing Amount and all Unpaid Reimbursement
      Obligations to exceed the Borrowing Base minus $5,000,000."

      Section 5. CONDITIONS TO EFFECTIVENESS. This Amendment shall not become
effective until the Agent receives the following:

      (a)   a counterpart of this Amendment signed by each of the Borrowers, the
            Agent and each of the Banks;

      (b)   an amendment fee of $112,500 paid by the Borrowers for the pro rata
            account of each Bank based on such Bank's Commitment Percentage;

      (c)   an opinion of counsel to the Borrowers, in form and substance
            satisfactory to the Agent; and

      (d)   such other documents, instruments, certificates or agreements as the
            Agent may reasonably require.

      Section 6. REPRESENTATIONS AND WARRANTIES. The Borrowers represent and
warrant that the representations and warranties of the Borrowers contained in
the Credit agreement and the other Loan Documents were true and correct when
made and continue to be true and correct on and as of the date hereof as if made
on the date hereof except to the extent of changes resulting from transactions
contemplated or permitted by the Credit Agreement or the other Loan Documents
and to the extent that such representations and warranties related expressly to
an earlier date and that no Default or Event of Default has occurred and is
continuing.

      Section 7. RATIFICATION, ETC. Except as expressly amended hereby, the
Credit Agreement and all documents, instruments and agreements related thereto,
including, but not limited to, the Security Documents, are hereby ratified and
confirmed in all respects and shall continue in full force and effect. The
Credit Agreement and this Amendment shall be read and construed as a
<PAGE>   4
single agreement. All references in the Credit Agreement or any related
agreement or instrument to the Credit Agreement shall hereafter refer to the
Credit Agreement as amended hereby.

      Section 8.   COUNTERPARTS.  This Amendment may be executed in one or more
counterparts, each of which shall be deemed an original but which together
shall constitute one and the same instrument.

      Section 9.   GOVERNING LAW.   THIS AMENDMENT SHALL BE GOVERNED BY, AND
CONSTRUED IN ACCORDANCE WITH, THE LAWS OF THE COMMONWEALTH OF MASSACHUSETTS
(WITHOUT REFERENCE TO CONFLICT OF LAWS).






                    [THE REMAINDER OF THIS PAGE HAS BEEN LEFT
                              INTENTIONALLY BLANK]
<PAGE>   5
      IN WITNESS WHEREOF, the parties hereto have executed this Amendment as a
document under seal as of the date first above written.


TRANSPRO, INC.


By:      /s/ TIMOTHY E. COYNE
         -----------------------------------
Name:    Timothy E. Coyne
Title:   V.P.


ALLEN HEAT TRANSFER PRODUCTS, INC.


By:      /s/ TIMOTHY E. COYNE
         -----------------------------------
Name:    Timothy E. Coyne
Title:   V.P.


AHTP II, INC.


By:      /s/ TIMOTHY E. COYNE
         -----------------------------------
Name:    Timothy E. Coyne
Title:   V.P.


EVAP, INC. (f/k/a EI Acquisition Corp.


By:      /s/ TIMOTHY E. COYNE
         -----------------------------------
Name:    Timothy E. Coyne
Title:   V.P.


GO/DAN INDUSTRIES, INC.


By:      /s/ TIMOTHY E. COYNE
         -----------------------------------
Name:    Timothy E. Coyne
Title:   V.P.


A/C PLUS, INC.


By:      /s/ TIMOTHY E. COYNE
         -----------------------------------
Name:    Timothy E. Coyne
Title:   V.P.
<PAGE>   6
BANKBOSTON, NA., individually and as Agent


By:      /s/ RICHARD D. BRIGGS, JR.
         -----------------------------------
Name:    Richard D. Briggs, Jr.
Title:   Director


PEOPLE'S BANK


By:      /s/ KEVIN. R. CALLAHAN
         -----------------------------------
Name:    Kevin R. Callahan
Title:   Vice President


THE BANK OF NEW YORK


By:      /s/ GERALDINE TURKINGTON
         -----------------------------------
Name:    Geraldine Turkington
Title:   Vice President


HARRIS TRUST AND SAVINGS BANK


By:      /s/ JEFFREY C. NICHOLSON
         -----------------------------------
Name:    Jeffrey C. Nicholson
Title:   Managing Director


BANK ONE, NA (Main Office Chicago)


By:      /s/ STEPHEN E. MCDONALD
         -----------------------------------
Name:    Stephen. E. McDonald
Title:   Senior Vice President