To provide scalable and simple Quality of Service(QoS) mechanism for multicast services,Probe-Based Multicast Admission Control (PBMAC) scheme was proposed. In this paper, PBMAC is studied and a so-called subsequent request problem is found in PBMAC, which degrades system performance significantly when the network traffic is heavily loaded. Based on the analysis on subsequent request problem, an Enhance PBMAC (EPBMAC) scheme is proposed, in which complementary probing is devised to solve the problem. Using a new metric of normalized requested equivalent link capacity, the performance of PBMAC and EPBMAC is analyzed and evaluated. Two implementations are proposed for incremental deployment. The paper finally introduces evaluation with packet-based simulations. Both analytical and simulation results show the significant improvement in performance.