Such system, project or combined system so purchased, constructed, improved, enlarged, extended or repaired shall remain subject to such statutory lien until payment in full of the principal of and interest upon the bonds. Any holder of any of such bonds or of any of the coupons representing interest accrued thereon may, either at law or in equity, by suit, action, mandamus or other proceedings, protect and enforce such statutory lien and may, by suit, action, mandamus or other proceedings, enforce and compel performance of all duties of the officials of the borrower, including the fixing of sufficient rates, the collection of revenues, the proper segregation of the revenues of the project or combined system and the proper application thereof. But such statutory lien shall not be construed to give any such bond or coupon holder authority to compel the sale of the project or combined system or any part thereof.