There are no trust funds.
The money is gone!
No institution can lend money to itself.
It is impossible for the SS system to loan money to the US Treasury.
There IS NO MONEY!
Why did you think I put surplus in quotes? Here is how the system works. SS is a pay-as-you-go system. Once all the revenue is collected from the payroll taxes and benefits are payed out, the money left over, i.e., the "surplus," is deposited in the general fund to be used as Congress sees fit.
In return, Treasury deposits the amount of the surplus into the SS Trust Fund in the form of non-market, interest bearing T-bills. They represent an unfunded liability, which is why they are included in the $11 trillion national debt as "Intragovernmental Holdings," which are different than the publicly-held portion of the national debt.
When payroll tax revenue is not sufficient to cover benefits, as happened in th3e early 1980s, the SSTF T-bills are redeemed by the USG to pay the benefits. Of course the USG must come up with the money from somewhere by increasing taxes, borrowing more money, or cutting spending or some combination thereof. The rationale for the SSTF is that it is better to owe the money to ourselves than borrow it elsewhere, like from the Chinese.
A few years ago I had an interesting discussion with Tom Savings, one of the SS Trustees, at a week long seminar on SS. He suggested that we abolish the SSTF and just include SS as a line item in the federal budget. It would be a more transparent and realistic way of dealing with SS. And the payroll tax would be seen for what it is, just another form of income taxes.
The problem with the entitlement programs except for Medicare Part B is that they are on automatic pilot and benefits paid out are not related directly to revenue received. COLAs and increasing medical costs are outpacing revenue and Congress can do little to stop it given the existing laws. And if we go into a period of high inflation, SS benefit payouts will skyrocket due to the COLA.