The custodian trustee shall concur in and perform all acts necessary to enable the trustees to exercise their powers of management or any other power or discretion vested in them, unless the matter in which he is requested to concur is a breach of trust or involves a personal liability upon him in respect of calls or otherwise, but unless he so concurs the custodian trustee shall not be liable for any act or default on the part of the managing trustees or any of them.