The only way to repair a flat roof, is to have years and years of experience diagnosing the possible points of entry and determining if it is even repairable in the first place.
Roofing Cement, by itself would only provide a 3 month to 2 year life cycle for sustainability, due to the cracking that will occur to the product when applied, it has a solvent in it to make it capable of being troweled in place.
For mastics such as this to last longer, a 3-ply method must be utilized, with the first coursing being the mastic, then either a cotton, fiberglass or polyester fabric being set into the mastic and then a surface skim coating of additional mastic.
The area being repaired must be free of dirt debris and and aggregate gravel surfacing, which must be spudded away carefully, so as to not create any further leak problems next to the repaired area.
If the roof has already been repaired with fresh mastic, (aka, roofing cement or plastic roofing cement), then any new roof surface coating will not adhere to that for any period of time, due to the solvents contained within starting to leech out of the product.
You would definitely be wasting your money on that type of repair venture.
Also, the type of tar that was used needs to be confirmed. If it is asphalt or bitumen, then re-roofing over the top is a viable option with the EPDM Rubber Roof. If, on the other hand, the product is Coal Tar Pitch, then the covering directly over the existing roof, without additional separator sheets will allow the chemicals to contaminate the newly installed roof membrane materials and they will decompose prematurely.
Ed